Update WFS usage docs

[KaiVolland]

This updates the docs for Filter usage with WFS.

I think that fes 2.0 is supported but needs a slightly different implementaton so i updated the docs.

You might want to check this, but this is how its working for me with WFS 2.0.2 and FES2

[geographika]

It looks like the server used in the example is no longer online: http://geoserv.weichand.de:8080/geoserver/wfs

https://www.weichand.de/

Die DemoServer (geoserv.weichand.de) stehen bis auf Weiteres nicht zur Verfügung.

I realise this server was used for the previous example, but if you are aware of another one we could use for the docs, and test this, that would be great.

[KaiVolland]

We could use this one https://sgx.geodatenzentrum.de/wfs_vg250?REQUEST=GetCapabilities&SERVICE=WFS but im not sure if I'll find the time to update the whole WFS section.

[pvgenuchten]

my suggestion would be to merge this, then create PR for url change

I tried https://sgx.geodatenzentrum.de/wfs_vg250?REQUEST=GetCapabilities&SERVICE=WFS, but it returns some security exception quite soon, in stead documenting with https://nationaalgeoregister.nl/geonetwork/srv/dut/catalog.search#/metadata/c46f802d-929e-4c59-b53f-0f74b14724e4 in #997