refactor: Updated build config and documentation theme

[frgfm]

This PR introduces the following modifications:

• moves all config files to pyproject.toml apart from .flake8 (no support yet cf. pyproject.toml (PEP 518) support PyCQA/flake8#234)
• removes old requirements.txt and dep verification job
• updates CI ref
• updates master branch ref to main
• updates README badges
• updates documentation theme
• moved collect_env from scripts to .github folder
• added black formatting and made mypy stricter
• fixed conda recipe

[codecov]

Codecov Report

Merging #213 (103d952) into main (9928a84) will increase coverage by 0.00%.
The diff coverage is 91.76%.

@@           Coverage Diff           @@
##             main     #213   +/-   ##
=======================================
  Coverage   93.50%   93.50%           
=======================================
  Files          56       56           
  Lines        3125     3126    +1     
=======================================
+ Hits         2922     2923    +1     
  Misses        203      203           

Impacted Files	Coverage Δ
holocron/models/classification/repvgg.py	88.50% <54.54%> (ø)
holocron/ops/boxes.py	98.24% <66.66%> (ø)
holocron/optim/wrapper.py	85.93% <68.75%> (ø)
holocron/nn/modules/loss.py	90.36% <71.42%> (ø)
holocron/trainer/classification.py	91.07% <71.42%> (ø)
holocron/nn/modules/lambda_layer.py	93.87% <72.72%> (ø)
holocron/nn/modules/downsample.py	92.75% <75.00%> (ø)
holocron/optim/functional.py	84.93% <75.00%> (ø)
holocron/optim/lars.py	76.36% <83.33%> (ø)
holocron/optim/tadam.py	78.33% <83.33%> (ø)
... and 34 more