Add an Index Page Based on Page Tags

[ilexp]

Summary

For quickly finding the right page, it might be good to have a central "index" page that lists all pages in the current versions docs grouped by each of the used tags.

Analysis

• Pages currently do not specify tags. For this to work, tags would need to be added first.
• The index page should sort its displayed tags alphabetically, and under each of them display a list of page links that match the given tag.
• Index pages could be included in the sidebar using regular sidebar logic by simply defining an empty index markdown file that uses a special HTML template. Similar to the current per-version "Home" pages.