fix: inject explicit HTML anchors in plumbing and porcelain Quick Index

GitHub generates anchors from the full header text (e.g. `#init--initialise-a-repository`), not from the command name alone. The Quick Index tables in both docs referenced short anchors (`#init`, `#hash-object`, etc.) that never resolved, breaking all in-page navigation.

Fix: prepend `<a id="command-name"></a>` before every section header covered by the Quick Index table — 31 anchors in porcelain.md and 24 in plumbing.md. Also simplify the plumbing.md table links from the broken long-form (`#hash-object----compute-a-content-id`) to the matching short form (`#hash-object`).