Updated docs to use main, removed master; updated release notes pointer

[dwk2]

1. Updated the mkdocs.yml to point our Release Notes to the newest 1.5.4 documentation.
2. Updated all of our documentation to refer to the main branch as our default branch; removed the term master from all current documentation.

• modified: docs/appendices/environment-files.md
• modified: docs/contributor-docs/editing-local.md
• modified: docs/contributor-docs/editing-online.md
• modified: docs/contributor-docs/making-pr-guide.md
• modified: docs/cookbook-recipes/previous-release-docs.md
• modified: docs/install/install-local-migrate.md
• modified: docs/install/install-local-new.md
• modified: docs/install/install-production-migrate.md
• modified: docs/install/install-production-new.md
• modified: docs/install/install-staging-migrate.md
• modified: docs/install/install-staging-new.md
• modified: docs/optional-components/automated-testing.md
• modified: docs/optional-components/varnish.md
• modified: docs/specifications/docker-containers-images.md
• modified: docs/specifications/dockerfiles.md
• modified: docs/update/update.md
• modified: mkdocs.yml

3. Removed 5 clearly obsolete files to trim documentation bloat:

• deleted: docs/contributor-docs/ISLE-v.1.1.2-buildnotes.md
• deleted: docs/contributor-docs/ansible-guide.md
• deleted: docs/contributor-docs/build-guide-v111.md
• deleted: docs/contributor-docs/build-guide.md
• deleted: docs/install/_obsolete_install-server.md