doc: add link headers - enhance support form ai agents

[BrunoCampana]

What problem does this PR solve?

This PR is part of an effort to make the docs website provide full support for AI agents.
In this context, one of the tools we are using for this is:
https://isitagentready.com/docs.qvac.tether.io?profile=content

The tool evaluates all the standards currently being used on the internet to provide support for AI agents to discover and consume the content of our docs website.
And one of the criteria suggested by the tool to improve support for AI agents is to add relevant links as headers.

How does it solve it?

We read the documentation of our CDN provider and created _headers files to point to the following resources:

• llms.txt
• llms-full.txt
• API reference
  Note: although several AI agents already look for the llms.txt/llms-full.txt files, this serves as reinforcement.

Our CDN platform will use the _headers file in the website's root directory to add the link headers.

PR preview / see it working

Check the headers in PR preview:
https://qvac-docs-staging-fazwv-commit-45deb41.kinsta.page

Example:

curl -sI 'https://qvac-docs-staging-fazwv-commit-45deb41.kinsta.page'/ | grep -i '^link:'

[kinsta]

Preview deployments for qvac-docs-staging ⚡️

Status	Branch preview	Commit preview
✅ Ready	Visit preview	Visit preview

Commit: 45deb41e289a1e7db8bb4e7d2bb78174e379b084

Deployment ID: 0fda1c54-f62d-4f88-b72b-062be26e061a

Static site name: qvac-docs-staging-fazwv

[github-actions]

Tier-based Approval Status

**PR Tier:** TIER1

**Current Status:** ✅ APPROVED

**Requirements:**
- 1 Team Member approval ✅ (1/1)
- 1 Team Lead OR Management approval ✅ (1/1)



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*This comment is automatically updated when reviews change.*

[BrunoCampana]

/review