docs(profiles): full user guide for profile distributions (#22017)

PR #20831 shipped the feature with a terse reference page. This adds a
proper user guide — ~570 lines of what/why/when/how with use-case
walkthroughs, lifecycle coverage from author through installer through
update, and recipe snippets for common workflows.

New page: website/docs/user-guide/profile-distributions.md

Sections:

* What this means — the before/after, side-by-side
* Why git, not tarballs or a custom format
* When to use a distribution (personal, team, community, product) and
  when NOT to (local backup, sharing credentials, sharing memories)
* The lifecycle — dedicated walkthroughs for authors (publish in 4 steps)
  and installers (install, check, update, remove)
* Use cases: personal sync, team internal bot, community publish,
  commercial product, ephemeral ops agent
* Recipes: pin a version, compare installed vs. latest, preserve local
  customizations through updates, force clean reinstall, fork-and-customize,
  test before pushing
* What is NEVER in a distribution (the user-owned exclude list verbatim)
* Security and trust model — what you are trusting, why cron is not
  auto-scheduled, the browser-extension analogy

Cross-linking:

* Added to sidebar under Getting Started, right after user-guide/profiles.
* Existing Profiles page ends with a Sharing profiles as distributions
  teaser that links here.
* The Distribution section of the reference page gets an admonition
  pointing newcomers here first. The reference stays as a CLI-flag
  lookup for people who already know what they want.

Validation:

* ascii-guard lint --exclude-code-blocks docs -> 0 errors.
* All internal links resolve to real pages.

website/sidebars.ts

@@ -28,6 +28,7 @@ const sidebars: SidebarsConfig = {
         'user-guide/configuring-models',
         'user-guide/sessions',
         'user-guide/profiles',
+        'user-guide/profile-distributions',
         'user-guide/git-worktrees',
         'user-guide/docker',
         'user-guide/security',