feat(config): add platform-specific filenames we'll look up

[jamietanna]

We should add support for a .{platform} folder for each of Renovate's supported Platforms.

This will require that [the filenames at the top of Configuration Options is amended.

Might want to be done after #40049.

Outstanding questions:

• Do we want to support .{platform}/renovate.json5, given we're recommending the use of JSONC?
• Do any of the platforms have a prefix they use for their own usage? I.e. .bitbucket for the bitbucket-server Platform?