[DOCS] Refactor expressions docs layout

[jaychia]

Flattened Expressions API docs

DataTypes and Series API docs have been unnested into just a flat class-level page

More todos for cleanups that can be done as follow-up PRs:

• We can probably cleanup our magic methods such as Expression.__add__ as a dedicated .add() method, and then document the use of the magic method in there.
• We will need much better docstrings (+ examples) on every method and function to make our doc really +++strong.

[codecov]

Codecov Report

Attention: 10 lines in your changes are missing coverage. Please review.

Comparison is base (854037e) 85.08% compared to head (be547e7) 85.60%.
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1816      +/-   ##
==========================================
+ Coverage   85.08%   85.60%   +0.51%     
==========================================
  Files          55       55              
  Lines        5713     6079     +366     
==========================================
+ Hits         4861     5204     +343     
- Misses        852      875      +23     

Files	Coverage Δ
daft/dataframe/__init__.py	100.00% <100.00%> (ø)
daft/dataframe/dataframe.py	88.26% <100.00%> (+0.06%)	⬆️
daft/__init__.py	24.24% <0.00%> (ø)
daft/expressions/expressions.py	90.86% <57.14%> (-1.83%)	⬇️

... and 11 files with indirect coverage changes