docs: #3613484 Add a docs folder for site builders, administrators and developers

Fixes #3613484.

Adds a docs/ folder so the module is documented for the people who actually use it, not only for developers reading the README.

What is in it

Page	For
docs/README.md	What the module does, and an index of the pages
docs/choosing-a-mode.md	Anyone using the assistant: picking a mode and what each one is for
docs/managing-modes.md	Site administrators: the form fields, the four dropdown placements, the permission
docs/developing-with-modes.md	Developers: the config entity, shipping modes in config or a recipe, the block and render element, the options endpoint, how steering works
docs/troubleshooting.md	The behaviours that look like faults and are not

Two screenshots under docs/images/, and a short Documentation pointer near the top of README.md.

Why the troubleshooting page exists

Three real behaviours read as bugs until explained, and all three cost time to work out:

• Both browser surfaces hide the dropdown when the only option would be "All, let the assistant decide", so a site with no saved modes shows no dropdown at all.
• In the "In the toolbar (compact)" placement the control is tied to the send button, which Drupal Canvas AI keeps hidden until there is something to send, so an empty message box shows nothing.
• The Modes listing reads "Prompt only" for a mode that steers by its system prompt addition alone, and "None" only for a mode that steers nothing.

Documentation only. No code, configuration, schema or test changes, so module behaviour is unaffected.

Checkpoints

• File an issue
• Addition/Change/Update/Fix
• Reviewed by a human
• Code review by maintainers