Fix for (#242): connectors not working brat json training issues

[percevalw]

Description

This should mostly fix #242.

• Fixes various errors in the docs
• Add an explanation for BRAT boolean attributes issue
• Makes edsnlp.data.write_json more verbose and smarter : when lines is left to None, it will now infer if the data should be written as a single JSONL file or as a directory of JSON files by default, based on the path argument being a file or not.

Checklist

• If this PR is a bug fix, the bug is documented in the test suite.
• Changes were documented in the changelog (pending section).
• If necessary, changes were made to the documentation (eg new pipeline).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (021f0c0) 96.70% compared to head (a68181c) 96.70%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #248   +/-   ##
=======================================
  Coverage   96.70%   96.70%           
=======================================
  Files         255      255           
  Lines        8372     8379    +7     
=======================================
+ Hits         8096     8103    +7     
  Misses        276      276           

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