feat(repo): prioritize well-known READMEs and optimize discovery (#38532)

This PR adjusts the repository README discovery logic to mirror GitHub's
priority order, while retaining support for Gitea's specific `.gitea/`
directory.

Previously, Gitea would prioritize a `README.md` at the root of the
repository over any READMEs in `.gitea/` or `.github/`. With this
change, well-known subdirectories are evaluated first when viewing the
repository root.

**New Priority Order:**
1. `.gitea/README.md`
2. `.github/README.md`
3. `/README.md` (root directory)
4. `docs/README.md`

This allows repository maintainers to use `.github/README.md` (or
`.gitea/README.md`) as the repository homepage without having to remove
or rename generic root `/README.md` files required by package managers
(like NPM, Crates.io, Docker, etc.).

---------

Signed-off-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

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