docs: give the user guide pages descriptive titles and link the README to the website (#167)

## Linked issue

Fixes #166

## Summary

- **User guide titles.** Every `docs/user-guide/*.md` H1 now says what
the page lets you do ("Send, reply, edit and react to Slack messages",
"Install SlackCLI on macOS, Linux and Windows", "Slack CLI for Claude
Code: plugin and skill"). The site takes its page titles from these H1s;
sidebar labels come from the manifest and are unchanged. No heading
anchors pointed at an H1, and `npm run check:links` passes on all 31
pages.
- **README.** The Docs nav link, the Quickstart, the "Built to be
scripted" section and the Documentation section now link to the
corresponding pages on slackcli.dev.
- **Landing page.** The H1 names the tool; the summary sentence in the
structured data matches the README and repository description; `sameAs`
points at the repository; the meta description is trimmed to display in
full.
- **Blog and plugin page descriptions** trimmed to display in full.

Documentation and website copy only. No code changes.

## Checklist

- [x] The linked issue is open and carries the `ready-for-pr` label
(constitution §1–2)
- [x] Change is focused on the linked issue — no unrelated edits
- [x] Tests added/updated, including edge cases (constitution §4) — not
applicable, docs and site copy only
- [x] `bun run type-check` and `bun test` pass locally — not applicable,
no code touched; site builds and link check passes
- [x] Pre-commit hooks are installed and passing — no `--no-verify`, no
`SKIP=` (constitution §5)
- [x] All commits are signed (constitution §5 — unsigned commits make
the PR unmergeable)
- [x] Documentation in `docs/` updated, added, or deleted as needed
(constitution §7)
- [x] No tokens, credentials, or user data handled insecurely

https://claude.ai/code/session_01GnfGN1dJ27YRHnMYymVrgs