docs: list every user-guide page in the documentation index (#169)

## Summary

Nightly documentation sync. Scope: one merged PR, **#168 — chore:
release v0.11.0**.

**#168 needs no documentation change.** It touches only `package.json`
(version bump) and `CHANGELOG.md` (promoting `[Unreleased]` to `##
[0.11.0]`). No command, flag, default, output shape or auth path moved,
and `docs/` names no version — the one version string in
`docs/development/build-and-release.md` is a placeholder in an example
`git tag` line and is deliberately not bumped per release.

## What this PR does fix

The audit of `docs/` against the tree turned up a real desync in the
top-level index. Five pages exist under `docs/user-guide/` and are
listed in `docs/user-guide/README.md`, but were never added to the table
in `docs/README.md`:

- `team.md` (#148)
- `usergroups.md` (#148, #156)
- `files.md` (#161)
- `emoji.md` (#149)
- `claude-code-plugin.md` (#163)

Each of those feature PRs shipped its own page and updated the
user-guide index, but not the top-level one. That table is not only the
GitHub entry point — `web/docs.manifest.mjs` publishes `docs/README.md`
as the site's `/docs/` landing page, so five command groups were
undiscoverable from it on <https://slackcli.dev> as well.

The rows are inserted in the order `docs/user-guide/README.md` lists
them, which is the order `web/docs.manifest.mjs` already follows for the
sidebar, so the three stay in step.

## Deliberately not changed

- Nothing for the release itself, per the reasoning above.
- No prose rewrites elsewhere. The rest of the user guide and developer
docs were spot-checked against `src/commands/`, `src/lib/workspaces.ts`
and the workflows and are accurate.
- No new page. Every command group already has one.

## Verification

- All 26 relative links in `docs/README.md` resolve to real paths.
- `git diff --stat origin/main...HEAD` shows `docs/README.md` only, +5
lines.
- Nothing outside `docs/` is touched, so `type-check` / `bun test` were
not required.

---

Opened by the scheduled documentation-sync job. Labelled
`no-issue-needed`: the changes being documented were already triaged in
the PRs above, so there is no separate feature to propose.