docs: response-rewrite plugin support for regular expressions

[NMichas]

Current State

response-rewrite plugin supports regular expressions, however the current doc page (https://apisix.apache.org/docs/apisix/plugins/response-rewrite) does not reflect that.

Desired State

Provide additional documentation/examples, showing the use of regular expressions. It would be beneficial if examples also showcase how multiple replacements for the same request can be performed, i.e.
<a href="/'> -> <a href="/some-other-path'>
<img src="/a.png'> -> <img src="/some-other-path-a.png'>

[spacewander]

PR is welcome!