Add support for a tag system on nix.dev

Current status

Currently pages on nix.dev can only be navigated to through links in the sidebar. Pages may also only appear a single time in the sidebar.

Problem statement

Since pages may only appear under a single topic, pages that may cover more than one topic must be arbitrarily placed under one of potentially many plausible topics. A better user experience would be if users could find all articles related to a particular topic regardless of where they appear in the site structure.

Approach

One method of adding this functionality is through a tag system. Tags may be added to the front-matter of each page. From the tags contained in each page a complete list of the tags may be generated during a build of the site. A page containing all of the tags can be generated from this list, and each tag listed on this page may link to a page containing all the pages that contain this tag.

There are Sphinx extensions that offer this functionality, namely sphinx-tags. Note that this Python module is not in nixpkgs and would need to be upstreamed in order to use it on nix.dev.

In a system with tags, a page may be placed under one canonical topic, but would also appear under the tag pages for any tags included in the page's front matter.

Suggested tag system

This is up for discussion, but an initial tagging system could look like this:

• nix-cli
• nix-lang
• nixos
• configuration
• tags for each language e.g. python, rust, etc
• etc

No files, tests, or entry points are named. Start by reviewing the nix.dev build configuration and the linked sphinx-tags extension, then determine how front-matter tags could generate a complete tag index and per-tag page listings. Done means the agreed tag system is implemented and pages are discoverable through their canonical topic and relevant tag pages.