Investigate if we can support both token and tokenless upload for codecov, and update docs

This adds support for tokenless uploads, we should decide whether to enable this at the same time

_Originally posted by @coretl in https://github.com/DiamondLightSource/python-copier-template/issues/213#issuecomment-2500883304_

No file or test is named; start with the referenced discussion in issue #213 and inspect the template's Codecov upload configuration and documentation. Determine whether token and tokenless uploads can coexist, then document the chosen behavior and define validation for the upload path.

索引モデルが issue の本文から書いたものです。