use a better example for the /content/ API endpoint

There are a number of optional features returned for /content/ objects via the Web API: blake2 checksum, filetype, language, license, etc. Also /raw/ download is available only for selected objects for now.

The example content object we use right now exposes almost no optional feature: cannot be downloaded, license has not been detected, etc. We should use a better example, that showcases all the additional information we have. Bonus point: it should be a "nice" piece of source code to look at.

Proposal: hello.c from (some version of) GNU Hello. But after making sure optional features for it are available. If they aren't/cannot be made available, we can try other options.

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Migrated from T753 (view on Phabricator)