[suggestion] add documentation for EXPath http-client module

[line-o]

Is your feature request related to a problem? Please describe.

The documentation and specification of the EXPath http-client module are both outdated. The documentation is sparse and could use some more examples. There are even cases where our implementation has diverged or simply is not usable as advertised.

Describe the solution you'd like

In order to provide our users with actionable information how to effectively send HTTP requests from within XQuery modules in exist I believe it is in our best interest to just create it ourselves.

Things to cover

• chunked="false"
• what src= actually does: loads files from filesystem, not resources from the database
• boundary hast to be set on content-type and for each http:multipart element

Describe alternatives you've considered

We could attempt to improve on the documentation on the project page. There is some discussion around that in expath/expath-http-client-java#9

I do still see the need of eXist-db specific documentation.

[line-o]

The only mention of the module I could find in our current docs is at the very end of the XQuery in eXist-db article under the headline Community Specifications and only provides a link the project home page.
https://exist-db.org/exist/apps/doc/xquery.xml?field=all&id=D3.61.7#D3.61.7