Add pydrake bindings for model directives.

[ggould-tri]

• Weld error support omitted for simplicity.
• Follow-up to "Model directives" scene assembly DSL #14038
• Completes Port "model directives" scene assembly DSL #13282

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This change is 

[ggould-tri]

+@EricCousineau-TRI
Follow-up to previous PR: Manual port of python bindings.
I am not a bindings expert and expect you will have plenty of objections here.

Reviewable status: LGTM missing from assignee EricCousineau-TRI(platform), needs at least two assigned reviewers (waiting on @EricCousineau-TRI)

[EricCousineau-TRI]

with a couple of nits - thanks!

I'm fine with serving as both feature and platform

Reviewed 4 of 4 files at r1.
Reviewable status: 2 unresolved discussions, needs at least two assigned reviewers (waiting on @EricCousineau-TRI and @ggould-tri)

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bindings/pydrake/multibody/parsing_py.cc, line 91 at r1 (raw file):

        py::arg("directives"), py::arg("plant"), py::arg("parser"),
        (std::string(parsing_doc.ProcessModelDirectives.doc) +
         "\nNOTE:  pydrake does not support `ModelWeldErrorFunction`.")

nit This should be compatible with Sphinx Napoleon:
https://www.sphinx-doc.org/en/master/usage/extensions/napoleon.html#docstring-sections

See example for a warning directive used in Drake here:

drake/bindings/pydrake/systems/lcm_py.cc

Lines 72 to 80 in ae06238

	constexpr char kLcmInterfaceSystemClassWarning[] = R"""(
	Warning:
	In C++, this class inherits both LeafSystem and DrakeLcmInterface. However,
	in Python, this only inherits from LeafSystem since our fork of pybind11
	does not support multiple inheritance. Additionally, it only exposes the
	constructor accepting a DrakeLcmInterface, so that it can still be used in
	interface code.
	)""";

See GSG styling (though not very comprehensive for directives...):
https://drake.mit.edu/styleguide/pyguide.html

(Will make a PR to try and link that in)

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multibody/parsing/BUILD.bazel, line 272 at r1 (raw file):

    data = glob(["test/process_model_directives_test/**"]),
    visibility = [
        "//visibility:public",

nit //visibility:public means other downstream consumer (e.g. Anzu, etc) can use this.

Consider the following options:

• //:__subpackages__ - Drake only (slightly more conservative)
• //bindings/pydrake:__subpackages__ - Bindings only (more conservative)
• //bindings/pydrake:multibody:__pkg__ - Multibody bindings only (most conservative).

[EricCousineau-TRI]

Reviewable status: 4 unresolved discussions, needs at least two assigned reviewers (waiting on @EricCousineau-TRI and @ggould-tri)

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bindings/pydrake/multibody/parsing_py.cc, line 67 at r1 (raw file):

    py::class_<parsing::ModelDirectives>(
        m, "ModelDirectives", parsing_doc.ModelDirectives.doc);
    m.def("LoadModelDirectives", &parsing::LoadModelDirectives,

All of these bindings should have docstrings available.

Can you add them in everywhere?

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bindings/pydrake/multibody/parsing_py.cc, line 69 at r1 (raw file):

    m.def("LoadModelDirectives", &parsing::LoadModelDirectives,
        py::arg("filename"));
    py::class_<parsing::ModelInstanceInfo>(m, "ModelInstanceInfo")

nit Per above defect, it'll be simpler if you just do a per-class alias:
Please consider aliasing using the provided template:
https://drake.mit.edu/doxygen_cxx/group__python__bindings.html#PydrakeDoc

Currently, you're breaking existing precedents (no alias or fully aliasing), I believe.

[ggould-tri]

+(status: single reviewer ok)

Reviewable status: 1 unresolved discussion (waiting on @EricCousineau-TRI)

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bindings/pydrake/multibody/parsing_py.cc, line 67 at r1 (raw file):

Previously, EricCousineau-TRI (Eric Cousineau) wrote…

All of these bindings should have docstrings available.

Can you add them in everywhere?

Done.

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bindings/pydrake/multibody/parsing_py.cc, line 69 at r1 (raw file):

Previously, EricCousineau-TRI (Eric Cousineau) wrote…

nit Per above defect, it'll be simpler if you just do a per-class alias:
Please consider aliasing using the provided template:
https://drake.mit.edu/doxygen_cxx/group__python__bindings.html#PydrakeDoc

Currently, you're breaking existing precedents (no alias or fully aliasing), I believe.

Not quite sure what you mean here, but I think I have done it now?

[EricCousineau-TRI]

Reviewed 3 of 3 files at r2.
Reviewable status: complete! all discussions resolved, LGTM from assignee EricCousineau-TRI(platform)

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bindings/pydrake/multibody/parsing_py.cc, line 69 at r1 (raw file):

Previously, ggould-tri wrote…

Not quite sure what you mean here, but I think I have done it now?

OK Yup - thanks!

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bindings/pydrake/multibody/parsing_py.cc, line 88 at r2 (raw file):

  constexpr char kWeldErrorDisclaimer[] = R"""(
    Note:

BTW Not sure if this will render well with the extra whitespace (given that it's getting appended, rather than being the docstring itself), but it's not a huge deal for me atm.