Mantra GSC new location (closes #891)

[phlobo]

Closes #891

Mantra GSC was moved from the original website to GitHub: https://github.com/mi-erasmusmc/Mantra-Gold-Standard-Corpus/tree/main

This PR makes the loader point to the new URL and creates a HF Hub version of the existing loader script for mantra_gsc.

If the following information is NOT present in the issue, please populate:

• Name: Mantra GSC
• Description: Multi-lingual Mantra Gold Standard Corpus
• Paper: https://academic.oup.com/jamia/article/22/5/948/930067
• Data: https://github.com/mi-erasmusmc/Mantra-Gold-Standard-Corpus/tree/main

Checkbox

• [x] Confirm that this PR is linked to the dataset issue.
• [x] Create the dataloader script hub/hub_repos/my_dataset/my_dataset.py (please use only lowercase and underscore for dataset naming).
• [x] Provide values for the _CITATION, _DATASETNAME, _DESCRIPTION, _HOMEPAGE, _LICENSE, _URLs, _SUPPORTED_TASKS, _SOURCE_VERSION, and _BIGBIO_VERSION variables.
• [x] Implement _info(), _split_generators() and _generate_examples() in dataloader script.
• [x] Make sure that the BUILDER_CONFIGS class attribute is a list with at least one BigBioConfig for the source schema and one for a bigbio schema.
• [x] Confirm dataloader script works with datasets.load_dataset function.
• [x] Confirm that your dataloader script passes the test suite run with python -m tests.test_bigbio_hub <dataset_name> [--data_dir /path/to/local/data] --test_local.
• [x] If my dataset is local, I have provided an output of the unit-tests in the PR (please copy paste). This is OPTIONAL for public datasets, as we can test these without access to the data files.

[leonweber]

@phlobo I cannot push mantra_gsc to the hub because of the following error:

- "language[0]" with value "en, fr, de, nl, es" is not valid. It must be an ISO 639-1, 639-2 or 639-3 code (two/three letters), or a special value like "code", "multilingual". If you want to use BCP-47 identifiers, you can specify them in language_bcp47.

Could you please open a PR to fix this?

[phlobo]

@leonweber thank you for taking care of this, please see #923