move the memverge doc from deployment/plugins to deployment/agent

[helenzhangyc]

Tracking issue

Describe your changes

Moved the MemVerge doc from deployment/plugins to deployment/agent

Check all the applicable boxes

• I updated the documentation accordingly.
• All new and existing tests passed.
• All commits are signed-off.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 58.90%. Comparing base (8090b16) to head (2e6fb47).
Report is 958 commits behind head on master.

❗ Current head 2e6fb47 differs from pull request most recent head 1d4ed64

Please upload reports for the commit 1d4ed64 to get more accurate results.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4073      +/-   ##
==========================================
- Coverage   58.95%   58.90%   -0.06%     
==========================================
  Files         618      618              
  Lines       52676    52575     -101     
==========================================
- Hits        31057    30968      -89     
- Misses      19140    19142       +2     
+ Partials     2479     2465      -14     

Flag	Coverage Δ
unittests	58.90% <ø> (-0.06%)	⬇️

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[pingsutw]

Actually, you don't need to create a folder for memverge agent. You could just add a memverge.rst under deployment/agents