Migration guide 2.x -> 3.0

[JrooTJunior]

Description

Added migration guide

Type of change

Please delete options that are not relevant.

• Documentation (typos, code examples or any documentation update)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• 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

[github-actions]

✔️ Changelog found.

Thank you for adding a description of the changes

[codecov]

Codecov Report

Merging #1143 (332586f) into dev-3.x (2ecf9ce) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##           dev-3.x     #1143   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          363       363           
  Lines         8979      8970    -9     
=========================================
- Hits          8979      8970    -9     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

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Files Changed	Coverage Δ
aiogram/client/session/base.py	100.00% <ø> (ø)
aiogram/exceptions.py	100.00% <ø> (ø)
aiogram/utils/formatting.py	100.00% <ø> (ø)
aiogram/dispatcher/router.py	100.00% <100.00%> (ø)