TINYDOC-3568 - Revitalise the Cloud quick start guides and scope the AI agents page

[kemister85]

Ticket: TINYDOC-3568

Site: [installing the official TinyMCE agent skill and letting an AI coding agent perform the setup via Cloud setup](https://pr-4316.tinymce-docs.iad.staging.tiny.cloud/docs/tinymce/latest/cloud-quick-start-ai/)
Site: describes the ways to put the current TinyMCE documentation in front of an agent

Changes:

Second iteration of the Build with AI work, based on the feedback on #4301. Based on hotfix/8/TINYDOC-3568 so the diff shows only the changes made in this iteration.

New AI quick start guide

• Adds cloud-quick-start-ai.adoc, an AI-agent path for the Cloud quick start: install the skill, prompt the agent, approve the domain, then configure further against live documentation.
• Both quick starts are interlinked at the top so readers landing on either one see the alternative.
• Linked from installation.adoc and installation-cloud.adoc as a second quick start card, and added to nav.adoc below the existing quick start guide.

AI agents page

• Scoped ai-coding-agents.adoc to documentation access only: the documentation MCP server, Context7, markdown pages, and llms.txt. The agent skill currently covers Cloud setups only, so the skill material now sits on the Cloud AI quick start where the claim holds.
• Per-agent MCP configuration snippets are collapsed into accordions.
• Adds a Context7 section.
• Retitled to "Documentation for AI agents" so it no longer competes with the quick start for the "Build with AI" name.

Cloud quick start guide

• Restructured to four steps: add the script, add the API key, initialize the editor, approve the domain. The API key step moved ahead of initialization so the no-api-key placeholder is explained the first time it appears.
• The editor example and the live demo now show a representative configuration (19 open-source plugins, a menu bar, and a full toolbar) instead of a bare tinymce.init({selector}). New cloud-quick-start live demo runs the same configuration as the example so the two cannot drift.
• Adds an "Approve the application domain" step, previously undocumented on this page.
• "Save the content from the editor" moves into Next steps, gated on productSource so only the two Cloud quick starts change. The npm, zip, php, and dotnet pages render as before.
• Refreshed the page metadata and description.

Notes for review

• @noxuhax The skill repository references use tinymce/tinymce-ai-skills. The tinymce/skills repository is currently private, so its install command and plugin marketplace commands would not work for public readers.
• The Claude Code plugin marketplace path was removed because tinymce/tinymce-ai-skills has no .claude-plugin directory. It can return once that lands.
• If we want to showcase premium features, we need to include the required options in the setup so they work out of the box for trial users.

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Pre-checks:

• Branch is correctly prefixed
• modules/ROOT/nav.adoc has been updated (if applicable).
• Files have been included where required (if applicable).
• Files removed have been deleted, not just excluded from the build (if applicable).
• Build passes without console errors, warnings, or issues.

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Review:

• Documentation Team Lead has reviewed.