docs: show extra-files syntax for generic files

[mrienstra]

Presently, finding the correct syntax is not easy, as it is not provided in any of the following:

• https://github.com/googleapis/release-please/blob/122820d/docs/customizing.md
• https://github.com/googleapis/release-please/blob/122820d/docs/manifest-releaser.md
• https://github.com/googleapis/release-please/blob/122820d/README.md

It can be found in the following places:

• https://github.com/googleapis/release-please/blob/122820d/schemas/config.json#L110-L118
• https://github.com/googleapis/release-please/blob/122820d/test/fixtures/manifest/config/extra-files.json

Related:

• https://github.com/google-github-actions/release-please-action/blob/ee9822e/README.md#adding-additional-files

---

Edit: to make it easier to skim over this, could be shortened to just:

{
  "extra-files": ["path/to/file.md"]
}