Improve root documentation overview and developer navigation

[EMaksy]

Description

This PR rewrites the root README.adoc to present the repository as the central documentation source for Trento and to make the published outputs, repository layout, and local preview workflows easier to understand.

It also updates the developer navigation so the root README appears in the docs hub as Trento Documentation.

What changed

• rewrote the root README.adoc from a narrow developer-docs page into a broader repository overview
• clarified that this repository contains:
  • user-facing Trento documentation
  • internal project documentation
  • Antora site assembly
  • custom UI
  • CI/build tooling
• documented the two published outputs:
  • Trento docs hub
  • official SUSE documentation
• added the current published version model:
  • main as pre-release
  • latest as current release
• added a clearer repository layout section for content/, trento/, trento-docs-site/, and trento-docs-site-ui
• separated documentation maintained in this repository from component documentation fetched from upstream repositories
• clarified that component documentation must be changed in the upstream source repository
• expanded local preview guidance for:
  • docs hub preview
  • SUSE HTML preview
  • SUSE PDF preview
  • full environment startup
• restored contribution notes for images under content/
• added links to contributor guides from the root README
• updated trento-docs-site/modules/developer/nav_developer.adoc so the page appears as Trento Documentation

Preview

Check out a build preview from my fork of this pr:
https://emaksy.github.io/docs/

https://emaksy.github.io/docs/developer/README.html and all the other documents.

[antgamdia]

I haven't had the chance to play around with the commands, but +1ing it as it improves the current documentation corpus and it's really valuable.
Thanks for the effort in putting it together!