docs: fix missing trailing periods in CONTRIBUTING.md

[amir-rezaei]

Description

This PR fixes missing sentence punctuation in CONTRIBUTING.md.

Details

1. Added missing trailing period at the end of the security process link sentence.
2. Added missing trailing periods to steps 1 and 2 under the Claiming an issue section.
3. Includes DCO sign-off.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.42%. Comparing base (e79be30) to head (525980a).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5961      +/-   ##
==========================================
- Coverage   72.42%   72.42%   -0.01%     
==========================================
  Files         734      734              
  Lines       75871    75871              
==========================================
- Hits        54950    54947       -3     
- Misses      17028    17034       +6     
+ Partials     3893     3890       -3     

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