Enhance the build by generating `llms.txt` file for LLMs consumption

[sneridagh]

This sphinx extension seems promising:

https://sphinx-llms-txt.readthedocs.io/en/latest/

We could easily integrate it with the current build.

Afterwards, we can point LLMs to there for their consumption, and integrating also with PloneMCP documentation specific tools.

https://github.com/adobe/react-spectrum/blob/main/packages/dev/mcp/shared/src/page-manager.ts#L17