[openstreetmap/openstreetmap-website] Clarify fixup commits and branching guidelines (PR #6594)

[Anshul Zilpe]

### Description
This PR updates `CONTRIBUTING.md` to address unclear guidelines regarding commit history and branching, as raised in #6592.

**Specific changes:**
* **Branching:** Added explicit advice in the "How to Contribute" section to create a feature branch and avoid working directly on `master`.
* **Commit History:** Moved the guidance on avoiding "fixup" commits from the "Large Pull Requests" tip to the main "Pull Requests" body. This clarifies that a clean commit history is expected for all contributions, not just large ones.

Fixes #6592

### How has this been tested?
This is a documentation-only change.
* I verified that the Markdown syntax renders correctly.
You can view, comment on, or merge this pull request online at:

  https://github.com/openstreetmap/openstreetmap-website/pull/6594

-- Commit Summary --

  * Clarify fixup commits and branching guidelines

-- File Changes --

    M CONTRIBUTING.md (4)

-- Patch Links --

https://github.com/openstreetmap/openstreetmap-website/pull/6594.patch
https://github.com/openstreetmap/openstreetmap-website/pull/6594.diff

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