[docs] Python documentation generation using Sphinx

[mugulmd]

Description

This PR introduces a documentation generation process using Sphinx for the Meshroom Python API.

The process also supports including the content of markdown files that belong to the project (such as README.md, INSTALL.md, etc.) for convenience using a custom directive called fetch_md.

The instructions for building the documentation are stored in docs/README.md.

Features list

• documentation generation process using Sphinx
• including the content of external markdown files
• custom documentation for nodes

Implementation remarks

• we use a custom module.rst template for the autosummary extension in Sphinx to have a more fine-grain control over the stub generation process and to be able to correct errors that occur during that process
• when importing markdown content with the fetch_md directive, the links based on relative paths are not valid anymore, so we try to re-link them (see the Relinker class), but for now this only works for images
• the global backend variable was creating import loops, so we removed it and isolated the backend selection in meshroom/common/__init__.py

Note

This PR is not about:

• writing the docstring of each python object in the project
• generating documentation for the QML code used for the UI