Created initial service navigation mock-up

[rmbielby]

Brief overview of changes

Having a quick stab at the service navigation. Ultimately required by shinyGovstyle, but creating in dfeshiny in case we want to demo / test it out here first.

Why are these changes being made?

To provide a fuller suite of GDS components.

Detailed description of changes

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Additional information for reviewers

Of interest for @pauljamesdfe & @cjrace

Still needs testing it actually works and some better fleshed out examples. I've basically just copied how we handle the footer links and gone with that. May be better ways to do it, but this seemed a nice simple starter for 10.

Issue ticket number/s and link

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 60 lines in your changes missing coverage. Please review.
✅ Project coverage is 63.33%. Comparing base (e402994) to head (a4bbed0).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #121      +/-   ##
==========================================
- Coverage   66.30%   63.33%   -2.98%     
==========================================
  Files          14       15       +1     
  Lines        1279     1339      +60     
==========================================
  Hits          848      848              
- Misses        431      491      +60     

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[cjrace]

I have made an exceptionally crude start already on shinyGovstyle on branch a little while back (GitHub Copilot helped mock out an initial structure), related issue - dfe-analytical-services/shinyGovstyle#99.

I think there's enough wider demand, and enough space to experiment in shinyGovstyle (e.g. could even flag the individual function as experimental) that we should move this work to there and go direct into shinyGovstyle rather than doing a DfE version first and needing to worry about porting over later, especially as we're admins / maintainers on there too now.

Very happy for you to take this code and put onto my branch or just start a new one on shinyGovstyle copying from what you've done here if what's on mine isn't helpful!