docs: document the provider category frontmatter

[atymic]

Documents the category frontmatter key that drives automatic listing, now that SocialiteProviders/Providers can add providers here on merge (SocialiteProviders/Providers#1465).

Also fixes a stale instruction: contributors were told to add the provider to monorepo-builder.yml, which no longer exists. That's split-overrides.json now, and only when the repo name differs from the directory name.