Fixes #427: Update Neovim CodeLens setup docs

[devin-ai-integration]

Summary

Updates the Neovim CodeLens setup docs to reflect the Neovim 0.12+ vim.lsp.codelens.enable(true) API instead of recommending repeated vim.lsp.codelens.refresh() autocmds.

Also documents:

• a minimal global enable snippet for users calling vim.lsp.enable('markdown_oxide')
• an optional LspAttach snippet scoped to markdown_oxide
• a note that vim.lsp.codelens.refresh({ bufnr = args.buf }) remains the Neovim <= 0.11 approach

Refs #427

Review & Testing Checklist for Human

• Verify the Neovim 0.12+ vim.lsp.codelens.enable(true, { bufnr = args.buf, client_id = client.id }) scoped example matches the current Neovim API and preferred style.
• Confirm both docs copies should remain manually synchronized.
• Optional: test the snippet in Neovim nightly/0.12 with markdown_oxide attached and confirm reference-count CodeLens appears.

Notes

Docs-only change; no Rust code changed.

Link to Devin session: https://app.devin.ai/sessions/c29484843aef4a05bac4df8517e0af80
Requested by: @Feel-ix-343

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[devin-ai-integration]

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.