Document the git.plugins category in the playbook schema

Add documentation for the new git.plugins category and its keys in the playbook schema. This category accepts a pre-defined map of key/value pairs, where the key is the name of the plugin and the value is the require request that specifies the location of the plugin code for Antora to require.

This category was added in order to specify plugins that the git client should use as an alternative to registering them with the git client directly.

Currently, there are two plugins, credential_manager and http. The first makes the authentication process pluggable and the second makes the HTTP operations pluggable.

Create a separate page to document these plugin keys and add a reference to that new page from the Configure Git page.