[directive] Collections

[gastmaier]

As demonstrated in branch mock (pseudo-code)
Collections are a documentation repo exclusive (probably) directive to aggregate pages across repos, in the same principle as custom doc yaml files, but simpler and stored into a json dict to be dynamically loaded and matched.

Suggestion syntax:

.. collection:: AD-APARD32690-SL

   hdl:
     projects/ad738x_fmc/index

   linux:
     iio/driver/ad9078

   no-OS:
     projects/ad7091r8-sdz

User visiting pages that belong to a collection, will see the collections that the page belong to in the sidebar:

If many collections are available, some may be in the collapsed state. Also have minor highlight for the current page in the collection, indicating "you are here", don't make it clickable, because the user may be in a sub-page.
an entry projects/ad738x_fmc includes all pages inside the path, with index inferred as top-level (assert if exists with interrsphinx?), while projects/ad738x_fmc/index will show on index page only.
As in the screenshot, the naming label will be automatic, unless using the Some name <path/to/page> toctree style overwrite.

In it's backend, does it need to use interphinx at all? or just use doc paths? Use it as assertion only? or resolve refs into doc path?

[gastmaier]

Base implemented at #41, hopefully for v0.4.19.

edit: released with v0.4.19, on the javascript side, will only work for documentation/collection.json, even though the directive can be used from any repo.
The reason is to avoid numbers-of-repos fetches for */collection.json.
Instead, wait for a global collector/aggregator.