Epic: v5 documentation

Track all v5 documentation work. Each sub-item below should land as
its own follow-up issue when we're ready to schedule it.

## Sub-items

### External

- [ ] learn.microsoft.com Core Tools refresh for v5 (landing +
reference pages updated, v4 marked legacy)
- [ ] Rebrand user-facing strings to "Azure Functions CLI" (CLI
strings, `User-Agent`, telemetry IDs, release notes)
- [ ] Update Azure Functions docs sample snippets to v5
(`--worker-runtime` to `--stack`, workload install steps)

### In-repo

- [ ] README rewrite for v5
- [ ] CONTRIBUTING refresh
- [ ] `repo-structure.md` refresh
- [ ] `building-a-workload.md` refresh
- [ ] v4 to v5 command map (in-repo)

## Definition of Done

- [ ] Each sub-item above has its own tracking issue opened
- [ ] All sub-items completed and merged

Start by separating the listed external and in-repo sub-items into follow-up tracking issues, including README.md, CONTRIBUTING, repo-structure.md, and building-a-workload.md. Use the v5 documentation checklist as the scope; done means every sub-item has its own issue and all listed work is completed and merged.

Written by the indexing model from the issue text.