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

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

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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