Update the compatibility matrix logic and documentation to reflect the actual
implementation: only Velocity performs the handshake validation, not Paper.

- Remove velocity-too-new and velocity-too-old from CompatibilityResult
- Simplify checkCompatibility() to check only if Velocity accepts Paper
- Update reason labels to clarify direction (Paper vs Velocity perspective)
- Add note distinguishing plugin version from protocol version
- Clarify Velocity-only validation in the protocol section
- Rewrite compatibility rules from Velocity's perspective

Paper only sends the handshake; it does not validate Velocity's version.
This aligns documentation with the actual behavior in
platform-velocity/.../PluginMessageHandler.kt.

Co-Authored-By: Claude <noreply@anthropic.com>

Add a dynamic compatibility matrix (GitHub Releases + ProtocolVersion.kt at
each tag) shown on the home page, download page, and a new reference doc.
Split protocol theory and rolling-update rules out of the Velocity feature
guide into the new compatibility reference, and simplify the download notice.

Also set explicit GitHub release titles in the Paper / Velocity workflows so
the awkward paper/v* tag prefix is not the user-facing label.

Co-Authored-By: Claude <noreply@anthropic.com>