docs: publish docs/ to the GitHub wiki and reorganise the user guide

- scripts/publish-wiki.sh publishes every page listed in SECTIONS, copies
  images, writes a sectioned sidebar and footer, and removes stale pages.
  --check fails on links to missing files or headings and on docs pages
  that are neither published nor excluded.
- New wiki.yml runs the check on pull requests and pushes to main, and
  publishes when release.yml calls it or when started by hand.
- Split USER_GUIDE.md into topic pages (files, synced folders, photos,
  sharing, computers, launcher and tray); add Getting started and FAQ.
- Make the COPR repository the main Fedora path, with update and
  uninstall steps for every package type.
- Fix the stale CAPTCHA note in CLI.md, document pdfs unlock and lock,
  and drop the broken AGENTS.md link from CONTRIBUTING.md.

docs/SHARING.md

@@ -0,0 +1,52 @@
+# Sharing
+
+Share files and folders with other people, by invitation or with a public link, and use what
+others share with you.
+
+- [Invite people](#invite-people)
+- [Create a public link](#create-a-public-link)
+- [See what you share](#see-what-you-share)
+- [Shared with you](#shared-with-you)
+
+## Invite people
+
+1. Select a file or folder and choose **Share…**.
+2. Enter one or more email addresses and choose **Can view** or **Can edit**.
+3. Send the invitation. People appear as members once they accept.
+
+The admin role, which may also manage the share, is available from the command line
+(`--role admin`).
+
+From a terminal:
+
+```bash
+pdfs share Documents/Plans alex@example.com --role editor
+pdfs members Documents/Plans          # members, pending invitations and the public link
+```
+
+`pdfs share-role` changes someone's role, and `pdfs unshare` removes one person or stops sharing
+altogether. See the [command-line reference](CLI.md#sharing).
+
+## Create a public link
+
+In **Share…**, create a public link. Anyone with the link can open the item. You can protect the
+link with a password and let it expire on a date. From a terminal, use `pdfs public-link create`
+and `pdfs public-link remove`.
+
+## See what you share
+
+**Shared by me** lists everything you share. Filter it by links, by people, or by expired links.
+Copy a link that is still valid, or stop several shares at once. `pdfs shared` prints the same
+list.
+
+## Shared with you
+
+**Shared with me** lists what other people share with you, who shared it, and what you may do with
+it. The same items are in the `Shared with me` folder inside `~/ProtonDrive`. Items where you are
+a viewer are read-only.
+
+- **Invitations** wait in a banner at the top of the page until you accept or reject them. From a
+  terminal: `pdfs invitations list`, then `accept ID` or `reject ID`.
+- **Saved public links** have their own view. `pdfs bookmarks` manages them.
+- **Leave…** a share you no longer need, or run `pdfs leave UID` with the ID from
+  `pdfs shared-with-me`.