Add a docs folder for site builders, administrators and developers

Problem/Motivation

The module has a README aimed at developers and a project page aimed at evaluators, and nothing in between. Site builders and administrators have nowhere to read how to create a mode, what each field on the form does, where the dropdown can sit, or why the dropdown is sometimes not there at all.

Several behaviours are deliberate but look like faults, and none of them are written down anywhere a user would look:

• Both browser-rendered chat 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.
• 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. The difference matters and is not explained.
• A mode never removes tools from the assistant. It only adds a steering instruction, which is reassuring to know and is currently undocumented.

Steps to reproduce

Not applicable. This is a documentation addition.

Proposed resolution

Add a docs/ folder to 1.0.x with one page per audience:

• docs/README.md: what the module does and an index of the pages.
• docs/choosing-a-mode.md: for anyone using the assistant.
• docs/managing-modes.md: for site administrators, covering the form fields, the four placements and the permission.
• docs/developing-with-modes.md: the config entity, shipping modes in config or a recipe, the block and render element, the options endpoint and how steering works.
• docs/troubleshooting.md: the behaviours above that look like faults and are not.

Add two screenshots under docs/images/, a short Documentation pointer near the top of README.md, and links to the docs from the project page.

Remaining tasks

• ✅ File an issue
• ✅ Addition, change, update or fix
• ✅ Documentation
• ❌ Reviewed by a human
• ❌ Code review by maintainers

User interface changes

None in the module. The project page gains links to the documentation.

API changes

None.

Data model changes

None.

Release notes snippet

Adds a documentation folder covering the module for site builders, administrators and developers, including a troubleshooting page for the behaviours that look like faults and are not.