📥 Add downloads to page/project frontmatter and site config

[fwkoch]

Previously, when you built a MyST site, all the exports defined on a page became Downloads and there was no other way to define additional or different downloads.

This PR adds downloads to page/project frontmatter as a place to specify (1) paths to local files and/or (2) export id values. Then, when you build your site, no longer do all the exports show up as downloads but only the specific downloads you specify.

This also makes improvements to site actions outlined here: #1048

[fwkoch]

TODO: Enable assigning a title on each download, as an alternative to the file name. We do not currently allow this for page exports, so maybe it can be an enhancement in a separate PR? But we do allow it for site actions so there is some precedent.

[stevejpurves]

Adding a title is definitely a good idea, currently with just exports themes are left to guess at what to put on the UI (https://github.com/executablebooks/myst-theme/blob/ef36c695035ae6895e7d022c9d00c5016d6c6c8e/themes/article/app/components/Downloads.tsx#L4) and with arbitrary downloads adding the filename is not a good option. So a title will benefit the themes well in both cases 👍

[fwkoch]

One last thing this needs is differentiation between downloads: undefined and downloads: [] - Pretty sure currently those both coerce to [] on the site config; we need to make sure we keep the differentiation.