docs: document voting, saving, profiles and settings on the Open WebUI Community page

[silentoplayz]

Summary

The Open WebUI Community page covered importing posts, sharing from an instance, model reviews and stats syncing, but not what people do on openwebui.com itself. This adds:

• Take part on the site: voting on posts and comments, commenting and replying, saving posts to the Saved tab, the Share menu, reporting posts and comments, following people, Notifications, and how mature content is shown.
• Post on the site: the Create Post form, posting to your profile or a community, editing and deleting your own posts, and joining or starting a community.
• Your profile and settings: what a profile shows, Edit Profile (banner image, picture, name, bio and link), each Settings tab (Account, Preferences, Email, API Keys and Subscription), account deletion and its 30-day grace period, and the theme switch.
• Report a problem with the site: Report Issue in the account menu leads to the community platform issue tracker, separate from the Open WebUI tracker and from reporting a post.

It also adds Notifications to the "What's on the site" table and lists the actions that need a signed-in account.

Related issue or discussion

None. This follows #1445, which added the page.

Checklist

• I have reviewed the relevant documentation and matched the existing style.
• This PR meets Open WebUI's contribution standards: it is accurate, relevant to users, narrowly scoped, maintainable, and not promotional content, advertising, lead generation, SEO placement, or a request to list a product, service, provider, integration, gateway, tool, or company primarily for visibility.
• I understand that PRs that do not meet these standards may be closed without review and will not be merged. Repeated, low-quality, off-topic, promotional, or intentionally misleading submissions may result in the contributor being blocked from future participation in Open WebUI repositories.

Notes for reviewers

I checked every control and label against openwebui.com while signed in, opening each menu and dialog and closing it without submitting anything. Three details come from the site's own interface text rather than a click: the comment sort options (Top, New and Old), the account deletion grace period, and that Edit and Delete show only on your own posts and comments unless your account is a moderator or admin. The page compiles as MDX.