chore(maindbv4): add a migration guide

[darrenvechain]

Description

Migration guide for mainDB v4

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• New and existing E2E tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have not added any vulnerable dependencies to my code

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 61.55%. Comparing base (571f607) to head (bb9a717).

Additional details and impacted files

@@                Coverage Diff                @@
##           release/maindbv4     #944   +/-   ##
=================================================
  Coverage             61.54%   61.55%           
=================================================
  Files                   217      217           
  Lines                 22635    22635           
=================================================
+ Hits                  13930    13932    +2     
+ Misses                 7583     7581    -2     
  Partials               1122     1122           

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