docs(skills): add COOKBOOK.md pattern for hand-written recipes alongside generated SKILL.md

[bishnubista]

Problem

SKILL.md files are auto-generated by gws generate-skills and contain API method listings. However, some services have non-obvious usage patterns that require hand-written guidance — particularly around:

• Google Docs tabs: reading specific tabs (includeTabsContent), creating tabs (addDocumentTab vs the invalid createTab), writing to specific tabs (tabId in location/range)
• Formatted content insertion: the insert-then-style pattern with insertText + updateParagraphStyle
• +write limitations: no tab targeting, plain text only

These patterns are hard to discover from gws schema output alone and typically require trial-and-error.

Proposal

Add a COOKBOOK.md file alongside SKILL.md in skill directories that need hand-written recipes:

```
skills/gws-docs/
SKILL.md ← auto-generated (unchanged)
COOKBOOK.md ← hand-written recipes (new)
```

Benefits:

• Safe from regeneration — gws generate-skills won't overwrite it
• Composable — other skills can adopt the same pattern as needed
• Discoverable — agents can check for COOKBOOK.md alongside SKILL.md

Optional follow-up: teach generate_skills.rs to append a "See also: COOKBOOK.md" link when one exists.

Scope

Starting with gws-docs as a proof of concept. The cookbook covers:

1. Tab CRUD operations (read, create, write, rename, delete)
2. Formatted content insertion (headings, bold/italic, paragraph styles)
3. Valid batchUpdate request types reference table
4. +write helper limitations and workarounds