docs: remove planning ai

[TomCools]

We no longer use PlanningAI as a term.
Removed the relevant page and folded the introduction into the "introduction" page. Quite obvious. :)

Clears up the navigation a bit more as well.

[Copilot]

Pull request overview

This PR removes the “PlanningAI” terminology from the documentation structure by deleting the dedicated concepts page and incorporating key introductory material into the main Introduction page.

Changes:

• Deleted planning-ai-concepts.adoc and removed it from the docs index and navigation.
• Reworked introduction.adoc to include planning-problem and constraints background previously covered elsewhere.
• Simplified the “Next” section to point readers to the Quickstart only.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
docs/src/modules/ROOT/pages/planning-ai-concepts.adoc	Removed the PlanningAI concepts page entirely.
docs/src/modules/ROOT/pages/introduction.adoc	Replaced PlanningAI reference with a broader introduction and added new sections on planning problems, constraints, and technology.
docs/src/modules/ROOT/pages/.index.adoc	Removed inclusion of the deleted planning-ai concepts page from the assembled guide.
docs/src/modules/ROOT/nav.adoc	Removed the PlanningAI concepts link from the navigation tree.

[triceo]

No redirect from the removed page?
Other than this and Copilot, LGTM.