Docs: Use sphinxcontrib-httpexample and convert existing examples

[lukasgraf]

Use sphinxcontrib-httpexample and convert existing examples in documentation to use the new directive.

Example Docs Build

Also

• Write example requests/responses to separate files
• Add HTTP/1.1 to the request/response line in order to make examples valid HTTP
• Dump request body (if there is one) to example files
• Include Content-Type header for requests that have a body
• Include Authorization header when dumping requests

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TODO (in follow-up PRs):

• Some examples still use literal sphinxcontrib-osexample base example requests. These didn't have a corresponding file in docs/source/_json that could be used with sphinxcontrib-httpexample, so those will need to be produced using a "test" in test_documentation.py first, and then converted
• Rename the folder with dumped requests/responses from _json to something more meaningful (_httpexamples?)
• When dumping request bodies, the JSON contained in them should be pretty printed beforehand

[datakurre]

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