Docs: highlight extension/server version matching and clarify opam install examples

[Maverick618]

When the VS Code extension and opam-installed vsrocq-language-server are on different versions, the language server may fail during initialization. This can cause commands like "Interpret to point" to silently do nothing(I just ran into this problem).

This PR clarifies the README by:

• Adding a "Version check" note recommending to keep extension and vsrocq-language-server versions aligned
• Showing how to install a specific language server version (e.g. vsrocq-language-server.2.3.4)
• Clarifying Rocq 9.x vs Coq 8.x package names (rocq-core vs coq-core)
• Noting that pre-release pins should also have matching extension versions