README update.

[cederom]

Summary

• Explain website organization, sources organization, local build and publishing process.

Impact

• Repo README / landing page.
• Better understanding of NuttX website and documentation organization.
• Contributing process.

Testing

• Tested on a fork repo.
• Please verify and request updates where necessary.

[cederom]

CI build fail is unrelated to this change. CI fix is here: https://github.com/apache/nuttx-website/pull/120.