Add T_AFTER CQL2 filter to documentation

[tfr42]

The documentation is missing the new T_AFTER filter in https://github.com/deegree/deegree-ogcapi/blob/main/deegree-ogcapi-documentation/src/main/asciidoc/usage.adoc#using-query-parameters

[tfr42]

With PR #163 the documentation was updated. It still needs some improvements to better understand the the supported functions and their operands.