Remove deprecated vignettes

[nikosbosse]

CLAUDE:

Summary

• Removes Deprecated-functions.Rmd and Deprecated-visualisations.Rmd vignettes, which documented functions that have already been removed from the package
• Adds a NEWS.md entry with a link to the git history so users can still find the old code if needed

Closes #1158

Test plan

• R CMD check passes without the removed vignettes
• pkgdown site builds without errors (no references to the removed vignettes in _pkgdown.yml)

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.98%. Comparing base (4a222da) to head (1415f0e).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1162   +/-   ##
=======================================
  Coverage   97.98%   97.98%           
=======================================
  Files          37       37           
  Lines        1984     1984           
=======================================
  Hits         1944     1944           
  Misses         40       40           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.