Improve docs by making example self-contained

[reborg]

A suggestion for several of the doc sections (for example https://docs.xtdb.com/clients/1.19.0/http/), it would be nice to always add the relevant require for the xt namespace or other namespaces mentioned in the examples. In general, the documentation section should be self-contained and run verbatim on a fresh REPL (that contains the relevant dependencies, which instead could be installed a one off in the introductory section and never mentioned again).