Makefile: Add clang formatting target

[hahnandrew]

• [x] Add a make target format to format files with clang
• [x] Update CONTRIBUTING.md

Fixes #566 Addresses @bjosv in https://github.com/valkey-io/valkey/pull/538#issuecomment-2134473903

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 70.35%. Comparing base (7795152) to head (e87a4b8). Report is 5 commits behind head on unstable.

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable     #924      +/-   ##
============================================
- Coverage     70.37%   70.35%   -0.03%     
============================================
  Files           112      112              
  Lines         61505    61505              
============================================
- Hits          43286    43270      -16     
- Misses        18219    18235      +16     

see 13 files with indirect coverage changes

[hahnandrew]

Closed in favor of git clang-format