Add config options to docs

[JohannesMP]

While writing #435 I noticed that, while Jekyll admin has dedicated documentation, it does not contain a page for which config settings can be used in _config.yml.

README.md provides a sample config for hiding links, but it is not clear if this is the complete set of config options currently available (since a repo's readme often only show a brief overview, reserving the complete extent of features for the docs).

Even if hidden_links is currently the only option for jekyll admin that you can use in _config.yml, having a dedicated config page in the docs would make it easier for users to verify.

[mertkahyaoglu]

Makes sense 👍 Could you open a PR for this?

[JohannesMP]

Done. Basically just contains what is in Readme.md right now.