docs: Consolidate "Developer's Guide" documents

Documentation/dev/git/README.md

Documentation/dev/git/dashboard.md

Documentation/dev/git/deprecation.md

Documentation/dev/git/download.md

Documentation/dev/release_process.md

Documentation/dev/testing.md

Documentation/docs/developers_guide/git/README.md

Documentation/docs/developers_guide/git/dashboard.md

Documentation/docs/developers_guide/git/deprecation.md

Documentation/docs/developers_guide/git/download.md

Documentation/docs/developers_guide/release_process.md

Documentation/docs/developers_guide/testing.md