old and incomplete docs

XBlocks are the heart of Open edX. Yet, somehow the documentation around it is not complete.
There are a lot of places that are referenced for future fixes or mentioned TODO. like:

• There's no information about Runtime services.
  https://docs.openedx.org/projects/xblock/en/latest/runtime.html#xblock.runtime.Runtime.service
  I found this in Jira but it's quite old: https://openedx.atlassian.net/wiki/spaces/AC/pages/161400730/Open+edX+Runtime+XBlock+API
• https://docs.openedx.org/projects/xblock/en/latest/xblock-tutorial/introduction.html#other-open-edx-resources

  You, the user community, can help update and revise this documentation project on GitHub.

• Nowhere inside the docs is mentioned about cookiecutter-xblock, but I think that's the standard approach to start with xblocks.
  https://edx.readthedocs.io/projects/xblock/en/latest/xblock-tutorial/getting_started/
• No reference to needs and wants decorators. (I know we can find the class method description https://edx.readthedocs.io/projects/xblock/en/latest/xblock.html#xblock.core.XBlock.needs but it's not clear for someone who's trying to develop an XBlock)

PS. We can add a tutor approach for installing xblocks as well.

@openedx/axim-engineering

Start with the XBlock Runtime API page and the xblock-tutorial pages linked in the issue, then compare them with the older Runtime XBlock API reference and cookiecutter-xblock repository. Define the documentation scope before editing: runtime services, cookiecutter-based setup, needs and wants decorators, and any Tutor installation guidance. Done means the selected topics are documented in the relevant tutorial or reference pages.