Document nixguix listing and loading behahior

The ingestion of the nixguix stack is a bit hard to follow. We have one lister which delegates loading to a bunch of core and vcs loaders.

Though new loaders are documented, they are not linked from any documentation page. Updating the nixguix listing and loading behavior will allow to reference those loader's documentation [2-9]

There exists a page dedicated for the nixguix which is not populated. That'd be a start [1]

List of loaders we can mention from that page are:

• ContentLoader [2]
• TarballDirectoryLoader [3]
• GitCheckoutLoader [5]
• HgCheckoutLoader [6]
• SvnExportLoader [8]

Refs. swh/meta#3781 (closed)

[1] https://docs.softwareheritage.org/user/loaders.html

[2] https://docs.softwareheritage.org/devel/apidoc/swh.loader.core.loader.html#swh.loader.core.loader.ContentLoader

[3] https://docs.softwareheritage.org/devel/apidoc/swh.loader.core.loader.html#swh.loader.core.loader.TarballDirectoryLoader

[4] https://docs.softwareheritage.org/devel/apidoc/swh.loader.git.loader.html#swh.loader.git.loader.GitLoader

[5] https://docs.softwareheritage.org/devel/apidoc/swh.loader.git.directory.html#swh.loader.git.directory.GitCheckoutLoader

[6] https://docs.softwareheritage.org/devel/apidoc/swh.loader.mercurial.directory.html#swh.loader.mercurial.directory.HgCheckoutLoader

[7] https://docs.softwareheritage.org/devel/apidoc/swh.loader.mercurial.loader.html#swh.loader.mercurial.loader.HgLoader

[8] https://docs.softwareheritage.org/devel/apidoc/swh.loader.svn.directory.html#swh.loader.svn.directory.SvnExportLoader

[9] https://docs.softwareheritage.org/devel/apidoc/swh.loader.svn.loader.html#swh.loader.svn.loader.SvnLoader