refactor: Refactored preprocessors and updated documentation

[fg-mindee]

This PR introduces the following modifications:

• moved task-specific (but not architecture-specific) preprocessors to their respective core.py
• improved documentation a bit

Here is how the landing page of the doc renders:

Any feedback is welcome!

[codecov]

Codecov Report

Merging #55 (a9b141f) into main (5afb0d7) will decrease coverage by 0.72%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #55      +/-   ##
==========================================
- Coverage   94.73%   94.01%   -0.73%     
==========================================
  Files          23       21       -2     
  Lines         703      685      -18     
==========================================
- Hits          666      644      -22     
- Misses         37       41       +4     

Impacted Files	Coverage Δ
doctr/models/detection/__init__.py	100.00% <ø> (ø)
doctr/models/preprocessor.py	96.66% <ø> (ø)
doctr/models/recognition/__init__.py	100.00% <ø> (ø)
doctr/models/detection/core.py	93.93% <100.00%> (+1.08%)	⬆️
doctr/models/recognition/core.py	94.73% <100.00%> (+2.73%)	⬆️
doctr/models/core.py	78.84% <0.00%> (-7.70%)	⬇️

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