docs(claude-md): fix Markdown rendering + clarify rst- prefix wording

Three rendering/clarity fixes flagged by Copilot review across the
post-flip CLAUDE.md trio (#1902/#1906/#1907):

1. **Triple-backtick examples in single-backtick code spans broke
   Markdown rendering.** The MyST fence example
   ```{cfgcmd} set system ...``` was wrapped in a single-backtick
   span, where the inner triple backticks confuse the parser. Switch
   to a four-backtick code span so the literal fence renders cleanly.
   Same fix applied to the "Configuration page structure" bullet.

2. **RST inline-literal example used confusing nested backticks.** The
   line "Inline code: \`\`\`\`command\`\`\`\`" parsed but
   was hard to read. Replace with a plain explanation:
   "Inline code: use double backticks (\`\`command\`\`)".

3. **Clarify that the `rst-` override prefix attaches to the basename,
   not the path stem.** The previous wording `docs/rst-<page>.rst`
   could be misread as a top-level prefix. The actual behavior in
   `scripts/swap_sources.py` is: for a page at
   `docs/automation/cloud-init.md`, the override file lives at
   `docs/automation/rst-cloud-init.rst` (basename-prefixed sibling).
   Spell that out with a concrete example.

Same fixes applied symmetrically across [#1902](https://github.com/vyos/vyos-documentation/pull/1902) (current),
[#1906](https://github.com/vyos/vyos-documentation/pull/1906) (circinus), and [#1907](https://github.com/vyos/vyos-documentation/pull/1907) (sagitta) — all three worktrees
back to byte-identical (md5 `d1ceaddc...`).

\xf0\x9f\xa4\x96 Generated by [robots](https://vyos.io)

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