Document the guides API, and move Guides beside Slides

The repo already enforces that every served route appears in
docs/api-inventory.json, and the guides endpoints did not - so the parity test
was failing. They are now listed with auth, params and a working example each,
beside knowledgebase because the two get read for the same reason.

docs/GUIDES.md covers what the inventory cannot: the block schema and why it is
blocks rather than HTML, the id rule that makes a tick rememberable, what the
server refuses and why validation happens at the door, the working-copy /
revision / published-pointer split that stops a live procedure changing under
somebody following it, and a worked example of creating and publishing a guide
from a script.

It also states plainly what does not exist yet - no importer, orphaned images
never pruned - and what converting existing documents will actually be like:
structured HTML and Markdown map onto block types nearly one for one, while a
plain-text runbook lands as paragraphs and needs marking up by hand afterwards.
Generated ids have to stay stable across re-imports, or every re-import resets
readers' progress.

Guides moves to position 8 in the sidebar, beside Slides, rather than down among
the reference lists in Information. A guide is something somebody goes and does
something with, not a catalogue they look things up in.

plugins/guides/plugin.py

@@ -75,13 +75,16 @@ class GuidesPlugin(BasePlugin):
         ]
 
     def get_navigation_items(self) -> List[Dict]:
+        # Position 8, beside Slides (7), rather than down in the Information
+        # section with the reference lists. A guide is something you go and DO
+        # something with, like a slide deck you put on a screen - not a
+        # catalogue you look things up in.
         return [
             {
                 'name': 'Guides',
                 'icon': 'book-open',
                 'route': '/guides',
-                'position': 36,
-                'section': 'information',
+                'position': 8,
             },
         ]