Update the metadata indexer documentation

We can have the following modifications in adding-support-for-additional-metadata page:

python3 -m swh.indexer.metadata_dictionary MyMapping path/to/input/file doesn't work. Replace with swh indexer mapping translate cff path/to/input/file
Whenever adding new mappings, it has to be mentioned in the MAPPING_NAME variable of the swh/indexer/storage/__init__.py file which isn't mentioned in the documentation. (though it didn't throw an error while testing or parsing) and in expected_output of test_cli_mapping_list
Mentioning a few examples about fields that are not string_fields
Elaborating how _traslate_dict function works. For example : it executes functions starting with 'normalize_'
Mention command : swh indexer mapping list-terms to display Supported CodeMeta terms
Add Youtube videos about JSONLD : JSON-LD Basics, JSON-LD: Core markup, Compaction and Expansion

Migrated from T3225 (view on Phabricator)

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