getting_started_api initialise.

Introduce the "getting started with the SWH API" document, as showed on [1]. This is basically a rst export of the document, with minor fixes to remove name/castalia header.

• [1] https://deepnote.com/project/Software-Heritage-aHgvv66LTAC1jgEYfe9elw/%2Fgetting_started_with_the_swh_api.ipynb

Test Plan

No test

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Migrated from D5784 (view on Phabricator)