fix: translation make commands

[irtazaakram]

Changes

• Fix extract_translations: drop --merge-po-files flag that crashes
  i18n-tools when an XBlock has no JS strings (html, lti, poll); merge
  manually with an existence check instead. Fix EXTRACT_DIR relative path
  so the django.po → text.po rename actually happens.
• Fix compile_translations: iterate over each XBlock instead of running
  once from the repo root.
• Fix build_dummy_translations: add missing extract_translations
  dependency so validate_translations works from a clean checkout.
• Fix XBLOCKS find: add -mindepth 2 to exclude the root-level combined
• Add combined django.po output: extract_translations now merges all
  per-xblock text.po files into
  xblocks_contrib/conf/locale/en/LC_MESSAGES/django.po for the
  openedx-translations pipeline (OEP-58).
• Fix .tx/config in all 8 XBlocks: correct the Transifex project from
  the non-existent p:xblocks to p:openedx-translations, and fix
  file_filter/source_file paths to match conf/locale/.
• Update README and docs with OEP-58 workflow, all Make targets, and
  troubleshooting guide.

Testing

• make extract_translations — produces text.po for all 8 XBlocks +
  combined django.po
• make validate_translations — full pipeline passes from clean checkout
• Simulated the openedx-translations GitHub Actions workflow locally
  (extract → compile → cleanup per-xblock dirs → find single EN_DIR →
  find django.po) — all steps pass

Related PR openedx/openedx-translations#70340
Most of the changes and Documentation is generated by Claude code.