docs(contributing): add documentation PR guidance targeting master

[luisangelrod]

Summary

Adds a new "Documentation Pull Requests" section to CONTRIBUTING.md explaining that docs-only fixes should target master instead of development. This resolves the ambiguity raised in #221.

The root cause: the published docs on readthedocs.io always point to master, so a fix merged only into development either waits behind the next release or forces merging unstable code into master.

Changes

• New ### Documentation Pull Requests subsection in the Documentation chapter
• Explains why docs PRs target master (not development)
• Provides a step-by-step snippet for checking out from master
• Includes a note for mixed code+docs PRs (use development as usual)

Closes #221

[jankapunkt]

@dhensby in regards to this, I would like to raise the question, if its still necessary to distinguish between master and development branches. I personally feel the overhead is not worth the benefits.

From what I see, we could be totally fine with

• master as base of truth for most recent development
• current major version releases are tags on the master branch
• superseded major versions having their own branches (4.x, 5.x etc.) and release tags on these branches

This would simplify the merging between major versions in case a fix needs to get reflected back to an older version.