std.http: add missing documentation and a few examples

[truemedian]

This adds the missing documentation for most of the previously undocumented functions, alongside a few example usages in certain scenarios.

This also removes the allocator parameter from Server and Server.init as it had no use and there are no plans to have a use for in the in near future.

[vexy]

Used to work on the same PR, just closed it a week ago. Anyhow, imho this looks good so far 👍

[andrewrk]

Nice work & apologies for the delay on the review & merge.