Add keeping fork up to date section in DG

[itsyme]

What is the purpose of this pull request?

• [x] Documentation update
• [ ] Bug fix
• [ ] Feature addition or enhancement
• [ ] Code maintenance
• [ ] DevOps
• [ ] Improve developer experience
• [ ] Others, please explain:

Overview of changes: Creates a new sub-section in the Workflow page of the developer guide about the recommended way to keep contributor's forks up to date with the main repo. I felt this was important as there are shortcuts like the 'Update branch' button in PRs that may be used by newer contributors who may not fully understand what happens under the hood. Keeping the fork in sync with the main repo would be a clean and straightforward way to keep the feature branches up to date.

Anything you'd like to highlight/discuss: Open for discussion :)

Testing instructions:

Proposed commit message: (wrap lines at 72 characters) Add keeping your fork up to date section in DG

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Checklist: :ballot_box_with_check:

• [ ] Updated the documentation for feature additions and enhancements
• [ ] Added tests for bug fixes or features
• [ ] Linked all related issues
• [x] No unrelated changes

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Reviewer checklist:

Indicate the SEMVER impact of the PR:

• [ ] Major (when you make incompatible API changes)
• [ ] Minor (when you add functionality in a backward compatible manner)
• [x] Patch (when you make backward compatible bug fixes)

At the end of the review, please label the PR with the appropriate label: r.Major, r.Minor, r.Patch.

Breaking change release note preparation (if applicable):

• To be included in the release note for any feature that is made obsolete/breaking

Give a brief explanation note about:

• what was the old feature that was made obsolete
• any replacement feature (if any), and
• how the author should modify his website to migrate from the old feature to the replacement feature (if possible).

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 48.90%. Comparing base (8a5688f) to head (712bdb0).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2453 +/- ## ======================================= Coverage 48.90% 48.90% ======================================= Files 124 124 Lines 5247 5247 Branches 1112 1112 ======================================= Hits 2566 2566 Misses 2373 2373 Partials 308 308 ```

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[itsyme]

I have added merge on top of just rebase for contributors to choose what they prefer