Docs: fine grained PAT's are now supported

[viceice]

We should remove the note about fine grained PAT support.

• docs: mention that Fine-grained PATs are not supported #778 (comment)
• docs: document support for fine-grained pat renovate#27346

[SergeyBear]

Not fully supported, because fine-grained PAT does not have access to Checks permissions (see discussions), so automerge will fail if ignoreTests set to false (by default)