UML-2140 add documentation only workflow

[jay-whitwell]

Purpose

When documentation only changes happen, we don't want to run the full pipeline and instead only checkout the md files

Fixes UML-2140

Approach

Add in the checks to the pull request, and if only md files are changed then run only the docs change

Checklist

• I have performed a self-review of my own code
• I have added relevant logging with appropriate levels to my code
• New event_codes have been documented on the wiki page
• I have updated documentation (Confluence/GitHub wiki/tech debt doc) where relevant
• I have added tests to prove my work
• I have added welsh translation tags and updated translation files
• I have run an accessibility tool on any pages I have made changes to and fixed any issues found
• I have notified the Interaction Designer of any content changes so that appropriate screenshots/flow diagram changes can be made
• The product team have tested these changes

[codecov]

Codecov Report

Merging #2461 (2e9b69e) into main (5521f91) will decrease coverage by 0.01%.
The diff coverage is n/a.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #2461      +/-   ##
============================================
- Coverage     91.94%   91.94%   -0.01%     
+ Complexity     1467     1466       -1     
============================================
  Files           279      278       -1     
  Lines          6655     6653       -2     
============================================
- Hits           6119     6117       -2     
  Misses          519      519              
  Partials         17       17              

Flag	Coverage Δ
use-an-lpa-admin	88.81% <ø> (ø)
use-an-lpa-api	96.80% <ø> (+0.18%)	⬆️
use-an-lpa-front	89.11% <ø> (-0.29%)	⬇️

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see 1 file with indirect coverage changes