Update Developer Hub with current developer resources

[AugmentSecurity]

Summary

Follow-up to the closed PR #341 and the maintainer feedback that the Developer Hub information itself needed a fuller update.

This revision updates wiki/Developer_hub.wikitext for #329 by:

• making the FreeCAD Developers Handbook the primary new-contributor entry point
• adding a current-resource section for the Developers Handbook, FreeCAD source repository, SourceDoc, Addon Academy and Lens documentation
• pointing build/setup guidance to the current Developers Handbook getting-started page
• adding current technical/source orientation and Addon Academy links to the modification section
• replacing the dated Module Developer's Guide chapter/PDF block with a shorter supplementary reference
• replacing the old FreeCAD 1.0 roadmap link with the current Developers Handbook roadmap plus the wiki roadmap
• removing the obsolete IRC/Gitter community row and adding GitHub issues/pull requests
• fixing the stale forum URL, malformed GitHub issue link, SourceDoc link and OpenCascade link/casing

Refs #329.

Validation

• Verified issue Updating the Developer Hub #329 is still open and labeled as a bounty/reward issue.
• Checked for competing Developer Hub PRs and did not find another active PR for Updating the Developer Hub #329.
• Checked the current linked resources:
  • https://freecad.github.io/DevelopersHandbook/
  • https://freecad.github.io/SourceDoc/
  • https://freecad.github.io/Addon-Academy/
  • https://github.com/FreeCAD/FreeCAD
  • https://github.com/FreeCAD/lens-docs
• Ran git diff --check origin/main...HEAD -- wiki/Developer_hub.wikitext.

No private data, credentials, signing material or payment details are included.

[Reqrefusion]

Please update the information; directing you to updates is not the purpose here.