docs(Flex): LLD and liquid classes

[emilyburghardt]

Overview

reworking the "what's new on Flex" section > "Flex-exclusive features" for a few reasons:

• the OT-2 and Flex comparison is no longer what's most important in the introduction. here, I've walked back the comparison text (but still include a small section to do so).
• this section can be useful to us to describe what's different/exclusive to Flex without comparison. This can also be a soft landing place for new features that aren't ready for an entire section, but will be one day (like LLD. After 8.8, this could very well get its own section).
  -also sprinkling throughout: LLD, liquid classes, a mention of liquid classes in QT. Trying to bridge the gap before some of these features get more new shiny things added to them and can have their own section (LLD mostly).

Test Plan and Hands on Testing

Changelog

Review requests

Risk assessment

[jwwojak]

Ye olde HTML suggestion included, free!

[ecormany]

LLD additions are good. The Introduction refactor needs some additional work, which you can either do here or move to a separate PR, since it's not focused on the LLD feature.

[jwwojak]

Just one comment, LGTM.

Still think you should use suspended hyphens though. 😜

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 25.48%. Comparing base (82fb268) to head (c744bc6).

Additional details and impacted files

@@            Coverage Diff             @@
##             edge   #19806      +/-   ##
==========================================
+ Coverage   24.23%   25.48%   +1.25%     
==========================================
  Files        3540     3540              
  Lines      298345   298348       +3     
  Branches    39863    39863              
==========================================
+ Hits        72298    76048    +3750     
+ Misses     226029   222276    -3753     
- Partials       18       24       +6     

Flag	Coverage Δ
app	3.11% <ø> (+2.18%)	⬆️

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