Fixed branch rename in documentation links

[jaapbrasser]

Fixed branch rename left over docs and links to edit

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I updated the docs, so no clue

Changes this PR brings

• Corrected the outdated "Master Branch" terminology to "Main Branch" in the documentation links.
• Corrected it in the readthedocs config so the edit buttons there point to the correct branch

Potential additional changes not added

Wasn't sure if this would break anything in the build projects

• GitVersion.yml refers to master
• tests/Project.Tests.ps1