docs: Expand SIG meeting welcoming language

[cijothomas]

Adds welcoming paragraph to the existing SIG meeting reference, matching the pattern recently merged in opentelemetry-js#6689 and several other SDK repos. Per discussion in community#1805.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.87%. Comparing base (cdadad6) to head (bd3ba34).

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #8383   +/-   ##
=========================================
  Coverage     90.87%   90.87%           
  Complexity     7970     7970           
=========================================
  Files           898      898           
  Lines         24040    24040           
  Branches       2397     2397           
=========================================
  Hits          21847    21847           
  Misses         1456     1456           
  Partials        737      737           

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[jack-berg]

Thanks!