Separate release notes for machine configuration
Prior to this change, the release notes for the Azure machine
configuration agent were kept in the `agents.md` file, colocating the
notes for releases on Windows and Linux in the same document. This made
it difficult for users to navigate to the correct release and wasn't
intuitive for discovery and reading.
This change:
- Separates the release notes by platform, making it easier to find the
correct release for a platform.
- Adds anchors to each article to simplify linking to a specific version
or release date.
- Replaces `agent.md` with `agent/index.md` as an overview to direct
readers to the platform-specific release notes. This obviates needing
a redirection as the URL remains valid.
- Fixes some minor grammar and formatting for the release notes.