Move intro docs from flytesnacks to flyte

[ppiegaze]

Covers DOCS-274 which is the first step of DOCS-273

Move the onboarding guide from flytesnacks to flyte repo without making any other visual changes to the docs site

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (aedde59) 58.51% compared to head (e4a9fac) 58.97%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4814      +/-   ##
==========================================
+ Coverage   58.51%   58.97%   +0.46%     
==========================================
  Files         493      644     +151     
  Lines       42771    55145   +12374     
==========================================
+ Hits        25026    32522    +7496     
- Misses      15749    20049    +4300     
- Partials     1996     2574     +578     

Flag	Coverage Δ
unittests	58.97% <ø> (+0.46%)	⬆️

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[cosmicBboy]

I updated conf.py to always use jupytext to convert .md files to notebooks. I also added back the yaml frontmatter, but just the minimal amount of config needed to render code. This will only be needed in .md files that contain {code-cell} ipython3 blocks, since the myst extension needs to know how to execute the code in these cells.