docs: document platformdirs change and platform-specific directory paths, fixes #7332

[mr-raj12]

Description

Document the platformdirs change introduced in PR #7300 for borg 2, as requested in #7332.

• Add platform-specific directory defaults table (Linux, macOS, Windows) to the environment variables docs
• Expand the platformdirs upgrade note in changes.rst with full macOS/Windows paths
• Add "Keyfile considerations when upgrading from borg 1.x" section to transfer docs, covering the key file location change on macOS/Windows and workarounds
• Update FAQ section titles and body text to be platform-aware instead of hardcoding Linux paths
• Fix hardcoded ~/.config/borg/keys references in repo-create, data-structures, and key docs

Fixes #7332

Checklist

• PR is against master (or maintenance branch if only applicable there)
• New code has tests and docs where appropriate
• Tests pass (run tox or the relevant test subset)
• Commit messages are clean and reference related issues