help(prompts): reorder LLM prompts, add chemical-formulas example, add hover descriptions

[ddehilster]

What

Follow-up to the LLM prompt library (#177):

• New first prompt — 01-chemical-formulas.md ("From scratch: chemical formulas"): a concrete, end-to-end example that gathers Wikipedia chemistry excerpts, then builds an NLP++ analyzer that finds chemical formulas and breaks each into its elements and atom counts, emitting JSON.
• Reordered so the worked example is first; the generic build prompt follows.
• Shorter titles — "Harden analyzer" and "Create Dictionaries & KBs".
• Hover descriptions — each prompt gets a <!-- desc: ... --> line. The Help-tree renders it as the tooltip; the prompt-opener strips it so it never lands in the text you paste into the LLM.

Prompt list after this change

1. From scratch: chemical formulas
2. NLP++ local install — build an analyzer
3. Harden analyzer
4. Create Dictionaries & KBs

Note

The tooltip rendering + desc stripping is a companion change in vscode-nlp (helpView.ts), submitted separately. This PR is the content only; it degrades gracefully on older extensions (the desc line is an HTML comment, so it stays hidden even if not stripped).

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