feat: support custom TOML manifest paths

[terror]

Resolves #5967

This diff allows --manifest-path to point at a Pixi manifest with any .toml filename, instead of only
accepting pixi.toml and pyproject.toml. Custom TOML manifests are treated as Pixi manifests when explicitly provided, and their lockfiles now use the same stem as the manifest, so custom-pixi.toml writes to custom-pixi.lock rather than pixi.lock.

The reference docs were updated to describe --manifest-path as accepting a manifest file or workspace directory. I also added a few tests that cover custom manifest provenance, explicit discovery, custom lockfile path derivation, and the end-to-end pixi lock --manifest-path custom-pixi.toml behavior.

[ruben-arts]

Maybe change this error aswell when it doesn't end in .toml:

> pixi install --manifest-path bla.bla
Error:   × unrecognized manifest file format. Expected either pixi.toml or pyproject.toml.

[ruben-arts]

Requesting @baszalmstra review. What do you think of the lockfile using the part before .toml as the part before .lock?