Add Airflow migration guide

[neverett]

What changes were proposed in this pull request?

Adds @pingsutw Airflow migration guide from https://github.com/flyteorg/flytesnacks/pull/1481 to Flyte, since the contents of the flytesnacks/docs directory have been moved to this repo.

Note: The link to the Airflow agent deployment guide is broken, since that guide is in a branch (see https://github.com/flyteorg/flyte/pull/4822).

Check all the applicable boxes

• [ ] All new and existing tests passed.
• [x] All commits are signed-off.

Related PRs

https://github.com/flyteorg/flytesnacks/pull/1481

Docs link

https://flyte--4871.org.readthedocs.build/en/4871/migrating_to_flyte/index.html

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 58.98%. Comparing base (33265b9) to head (66af8ce). Report is 13 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4871      +/-   ##
==========================================
+ Coverage   58.97%   58.98%   +0.01%     
==========================================
  Files         645      645              
  Lines       55561    55542      -19     
==========================================
- Hits        32766    32764       -2     
+ Misses      20200    20187      -13     
+ Partials     2595     2591       -4     

Flag	Coverage Δ
unittests	58.98% <ø> (+0.01%)	:arrow_up:

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