Updates instructions and screenshots for the new weblate domain and git repository

[kushaldas]

Status

Ready for review.

Description of Changes

Fixes #3929

This updates the documentation for i18n where we now point to the right repository and commands to sync
the weblate from development branch, and also vice-versa.

I have also updated the screenshots.

Testing

I am going to update the testing steps below.

Checklist

If you made changes to the server application code:

• Linting (make ci-lint) and tests (make -C securedrop test) pass in the development container

If you made changes to securedrop-admin:

• Linting and tests (make -C admin test) pass in the admin development container

If you made changes to the system configuration:

• Configuration tests pass

If you made non-trivial code changes:

• I have written a test plan and validated it for this PR

If you made changes to documentation:

• Doc linting (make docs-lint) passed locally

[codecov-io]

Codecov Report

Merging #3939 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #3939   +/-   ##
========================================
  Coverage    84.55%   84.55%           
========================================
  Files           43       43           
  Lines         2726     2726           
  Branches       296      296           
========================================
  Hits          2305     2305           
  Misses         354      354           
  Partials        67       67

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1b5ea7d...3edd6fd. Read the comment docs.