Improve Contributing docs and set a release schedule (#37109)

This PR updates `CONTRIBUTING.md` for clarity (code review, maintainers,
PR workflow)

## Suggestion

- majors about every **three months**, with a more predictable cadence
from **v1.26** onward.
- target dates such as **v1.26.0** (April 2026), **v1.27.0** (June
2026), **v1.28.0** (September 2026), **v1.29.0** (December 2026).
- announce feature freeze **two weeks** before each release.

## Other doc changes

- Reviewing PRs: separate guidance for reviewers vs authors; small edits
to maintaining PRs, merge queue, commit messages, co-authors.
- Maintainers: clearer subsections; links to GitHub Docs for 2FA / GPG.
- Split the Contributing.md into more useful markdown files

---------

Signed-off-by: Nicolas <bircni@icloud.com>

docs/guideline-frontend.md

@@ -0,0 +1,17 @@
+# Frontend development
+
+This document covers frontend-specific contribution expectations. For general contribution workflow, see [CONTRIBUTING.md](../CONTRIBUTING.md).
+
+## Dependencies
+
+For the frontend, we use [npm](https://www.npmjs.com/).
+
+The same restrictions apply for frontend dependencies as for [backend dependencies](guideline-backend.md#dependencies), with the exceptions that the files for it are `package.json` and `package-lock.json`, and that new versions must always reference an existing version.
+
+## Design guideline
+
+Depending on your change, please read the
+
+- [backend development guideline](https://docs.gitea.com/contributing/guidelines-backend)
+- [frontend development guideline](https://docs.gitea.com/contributing/guidelines-frontend)
+- [refactoring guideline](https://docs.gitea.com/contributing/guidelines-refactoring)