Add meeting and membership session to contributing docs

[khanhtc1202]

What this PR does:

as title

Why we need it:

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

• How are users affected by this change:
• Is this breaking change:
• How to migrate (if breaking change):

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 28.67%. Comparing base (660e79e) to head (ff54b21).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6142   +/-   ##
=======================================
  Coverage   28.67%   28.67%           
=======================================
  Files         535      535           
  Lines       57335    57333    -2     
=======================================
  Hits        16442    16442           
+ Misses      39615    39613    -2     
  Partials     1278     1278           

Flag	Coverage Δ
.	23.27% <ø> (+<0.01%)	⬆️
.-pkg-app-pipedv1-plugin-analysis	62.85% <ø> (ø)
.-pkg-app-pipedv1-plugin-kubernetes	62.30% <ø> (ø)
.-pkg-app-pipedv1-plugin-kubernetes_multicluster	67.48% <ø> (ø)
.-pkg-app-pipedv1-plugin-scriptrun	54.83% <ø> (ø)
.-pkg-app-pipedv1-plugin-terraform	38.31% <ø> (ø)
.-pkg-app-pipedv1-plugin-wait	35.51% <ø> (ø)
.-pkg-app-pipedv1-plugin-waitapproval	68.62% <ø> (ø)
.-pkg-plugin-sdk	50.98% <ø> (ø)
.-tool-actions-gh-release	19.23% <ø> (ø)
.-tool-actions-plan-preview	25.30% <ø> (ø)
.-tool-codegen-protoc-gen-auth	0.00% <ø> (ø)

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[t-kikuc]

I guess the joining timing should be flexible once they have at least once attended.
Contributors are not always available to attend the meeting.