Add Available facets page in documentation

Problem/Motivation

Range slider was just added in #3589695: Add exposed range slider support for facets_exposed_filters (processor + BEF widget) which is awesome.

As a user of the Facets module, it would be nice if the available facets were listed somewhere, so you know which ones are available via Facets, and when you need to use a contrib module.

Planned facets, or facets being worked on are also of interest, and could be listed as well.

Steps to reproduce

Proposed resolution

Perhaps it could be considered to add a new "Available facets" page on https://project.pages.drupalcode.org/facets/, where the "Range slider" and all the other currently available facets could be listed?

Planned facets, or those under way could also be listed under a "Planned Facets" section.

Remaining tasks

User interface changes

API changes

Data model changes