Add documentation and deprecate to_bigint and to_biguint

[Juan-M-V]

TITLE

Description

Description of the pull request changes and motivation.

Checklist

• Linked to Github Issue
• Unit tests added
• Integration tests added.
• This change requires new documentation.
  • Documentation has been added/updated.

[codecov]

Codecov Report

Merging #757 (7cb731a) into main (0991fb4) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #757   +/-   ##
=======================================
  Coverage   97.12%   97.12%           
=======================================
  Files          69       69           
  Lines       28800    28825   +25     
=======================================
+ Hits        27972    27997   +25     
  Misses        828      828           

Impacted Files	Coverage Δ
..._processor/builtin_hint_processor/secp/ec_utils.rs	98.98% <ø> (ø)
...ocessor/builtin_hint_processor/secp/field_utils.rs	98.73% <ø> (ø)
...processor/builtin_hint_processor/secp/signature.rs	98.67% <ø> (ø)
..._processor/builtin_hint_processor/uint256_utils.rs	100.00% <ø> (ø)
src/vm/runners/cairo_runner.rs	97.89% <ø> (ø)
felt/src/lib.rs	75.19% <100.00%> (+0.53%)	⬆️
...int_processor/builtin_hint_processor/math_utils.rs	97.37% <100.00%> (+<0.01%)	⬆️
...rocessor/builtin_hint_processor/secp/secp_utils.rs	100.00% <100.00%> (ø)
src/vm/runners/builtin_runner/ec_op.rs	98.91% <100.00%> (+<0.01%)	⬆️

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