[CELEBORN-1458][DOC] Introduce decommissioning document

[AngersZhuuuu]

What changes were proposed in this pull request?

Introduce decommissioning document to provide users with introduction of worker decommissioning.

Why are the changes needed?

Users should know operation of worker decommissioning maintenance.

Does this PR introduce any user-facing change?

No.

How was this patch tested?

No.

[AngersZhuuuu]

ping @SteNicholas @RexXiong @waitinfuture

[SteNicholas]

@AngersZhuuuu, thanks for udpates. I left the minor comments for the changes. You could rename decommission.md to decommissioning.md which follows the pattern of upgrading.md. PTAL.

[AngersZhuuuu]

@AngersZhuuuu, thanks for udpates. I left the minor comments for the changes. You could rename decommission.md to decommissioning.md which follows the pattern of upgrading.md. PTAL.

Done

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 40.49%. Comparing base (50eaaf7) to head (a5ff5de).
Report is 15 commits behind head on main.

❗ Current head a5ff5de differs from pull request most recent head dc04a48

Please upload reports for the commit dc04a48 to get more accurate results.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2554      +/-   ##
==========================================
- Coverage   40.58%   40.49%   -0.08%     
==========================================
  Files         222      222              
  Lines       14261    14289      +28     
  Branches     1284     1291       +7     
==========================================
- Hits         5786     5785       -1     
- Misses       8145     8173      +28     
- Partials      330      331       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[RexXiong]

Perfer change Decommissioning to scaling include scale up/scale down. For scaling up, Celeborn only needs additional workers to be added. For scaling down, decommissioning is required first.

[RexXiong]

merge to main(v0.6.0)