References for config file values

[hoefling]

This is another suggestion for adding crossrefs to the docs, a follow-up of #7652 (crossrefs to standard library items) and #7784 (crossrefs for command line options), only now for the config file values.

Since Sphinx doesn't define any config-specific role, I have added the special confval directive and role. The .. confval:: directive accepts two optional info fields, :type: and :default:. For example,

.. confval:: verbosity

    :type: integer
    :default: 0

    Controls how much debug output will be generated.  Higher numbers are more verbose.

renders as

The :confval: role adds a crossref to the config value in a similar fashion as the :option: role for command line options. Example:

See the :confval:`verbosity` configuration option for more details.

renders as

If you like this suggestion, I will add another PR that adds crossreferences to the config file values throughout the docs.

[hoefling]

This newline is not related to this PR's topic, but it fixes a small rendering issue. Right now on current master, the list is rendered like this:

This is because the list items are too deeply indented. With a proper separation of the list and the preceding paragraph, it becomes

[JukkaL]

Thanks! It's to be able to add links to config file options.

[hoefling]

@JukkaL awesome, thanks! I will then prepare (or better to say, blow the dust off of) an another PR that will add crossrefs for config values in the current docs.