Docs contrib guide and localisation updates

• Adds a Contrib guide for the community

  • Standard English UI contributions
  • Localisation for more languages

• Updates for localisation

  • dev guide (draft for @grgml review and add relevant sections)
  • user guide

Still to do:

• Add QA guide from @bela
• Contributing feature requests and bug reports
• Add new section "Get Involved" to nav and sidebar
• order files in "Get Involved"