The metadata is copied from the documentation's plugin_matrix, which is
going to be replaced by a sphinx extension which parses the metadata
in each plugin file and dynamically generates the plugins document.
Metadata tests will be added with sphinx extension.
TODO:
- Add $description metadata tags. Some of the plugin descriptions can be
  found in the git history, while others have to be retrieved from
  issues and pull requests on GitHub, or they have to be looked up.
- Add URLs to metadata which were previously missing in the
  plugin matrix due to space constraints (see the matcher regexes).

and fix links in other documents

Implement custom "plugins" sphinx extension which parses metadata from
plugin files and which outputs a list of plugins with datalists that
include the annotated metadata in addition to plugin arguments and
links to plugin files + issues on GitHub.
Add plugin metadata linting tests and remove old plugin matrix tests.

Implement custom "plugins" sphinx extension which parses metadata from
plugin files and which outputs a list of plugins with datalists that
include the annotated metadata in addition to plugin arguments and
links to plugin files + issues on GitHub.
Add plugin metadata linting tests and remove old plugin matrix tests.