Deprecate or remove path in front matter

[bep]

We added it in v0.123.0 and it's not documented, so it should not be in widespread use.

So, it works, but the main problem with it, which I'm not prepared to spend time fixing, is the partial rebuild on content change logic.

This was added to prepare for "data from pages", which was a good idea, but for content filed (e.g. mypage.md), it's not.

[jmooring]

I'd say remove it without deprecation. The kind and lang front matter values not documented either, so if you need to...