Update filtering API documentation

[turt2live]

Rendered: see 'docs' status check

This PR may be best to review commit-by-commit.

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Fix naming of the Filter schemas
EventFilter !== Filter
Fixes #1509

Update room_event_filter.yaml to use the OpenAPI allOf definition

Mark the filter_id in the response of POST /filter as required

Clean up examples in filter.yaml

Define the default for the contains_url filter param
Fixes #1553

[turt2live]

@ara4n I'm particularly interested if this helps/harms/has no effect on #1508. Without major changes to the build/rendering process, I don't think it can be made much clearer.

[ara4n]

i think this almost fixes #1508, but it still needs CSS to actually make the tables & headings legible. That's an entirely separate problem though (although one i'd like to try to fix this week somehow!)

[ara4n]

lgtm

[turt2live]

I've updated the title of #1508 to be more generic in respect to fixing it spec-wide. I'd say it's unlikely to get done before Friday, however there is some light at the end of the r0 tunnel. I'll put on the r0 list under the cosmetic fixes (as that's what it is), although all the cosmetic stuff is under the heading "if we have time...".