Add field "Include in llms.txt" for all content types

Problem/Motivation

In this issue 🌱 [META] AI LLM Optimized Content Active it was discussed how to add more control over the content that is exposed in llms.txt file.

User story

As a content editor, I want to categorize my content for AI systems so that relevant articles appear in the site's llms.txt file and can be discovered by language models and AI agents.

Acceptance criteria

Content categorization

• Given I am creating or editing a node, when I view the content form, then I can see an "LLMs.txt content section" field
• Given the "LLMs.txt content section" field exists, when I click on it, then I can select from available taxonomy terms (sections)
• Given I select a section, when I save the content, then the content is tagged with that taxonomy term

Content inclusion behavior

• Given I have tagged content with an LLMs.txt section, when the llms.txt file is generated, then my content appears under the corresponding section header
• Given I have not tagged content with any section, when the llms.txt file is generated, then my content does not appear in the file
• Given I remove the section tag from existing content, when the llms.txt file is regenerated, then that content no longer appears in the file

Content organization

• Given multiple pieces of content are tagged with the same section, when viewing the llms.txt output, then they are grouped together under that section's header
• Given a section has a description, when viewing the llms.txt output, then the description appears before the list of content links in that section