docs: update docs/getting-started/collections.md experimental row counts after caveman skill addition

[github-actions]

Component: Documentation

Bug Description:
docs/getting-started/collections.md has stale artifact counts for the experimental collection row. PR #1861 added the caveman skill to the experimental collection and updated collections/experimental.collection.yml, but the Available Collections table in the documentation was not updated.

Specific stale values vs. actual counts from collections/experimental.collection.yml:

Field	Current (stale)	Actual (from YAML)
Agents	3	3 ✓
Prompts	0	2
Instructions	3	11
Skills	2	7

Expected Behavior:
The experimental row in docs/getting-started/collections.md should reflect current artifact counts from collections/experimental.collection.yml.

Steps to Reproduce:
PR #1861 (feat: add experimental Caveman response style skill) added .github/skills/experimental/caveman to the experimental collection. The collection YAML was updated but docs/getting-started/collections.md was not.

To verify:

grep "kind: agent" collections/experimental.collection.yml | wc -l      # returns 3
grep "kind: prompt" collections/experimental.collection.yml | wc -l     # returns 2
grep "kind: instruction" collections/experimental.collection.yml | wc -l # returns 11
grep "kind: skill" collections/experimental.collection.yml | wc -l      # returns 7

Additional Context:

• Changed code: collections/experimental.collection.yml
• Stale doc: docs/getting-started/collections.md (Available Collections table, experimental row)
• Related open issues: docs: update docs/getting-started/collections.md artifact counts for PR #2062 skill consolidation #2078 covers project-planning and hve-core-all row counts; docs: update docs/getting-started/collections.md hve-core row counts after PR Walkthrough addition #2080 covers hve-core row counts; docs: update docs/getting-started/collections.md for jira-setup prompt addition #1869 covers jira row counts — all in the same file
• The prompt and instruction counts are also stale (pre-existing drift), not just the skill count from this PR

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