Remove site:run goal in order to reduce amount of dependencies

### New feature, improvement proposal

The mojo https://github.com/apache/maven-site-plugin/blob/master/src/main/java/org/apache/maven/plugins/site/run/SiteRunMojo.java depends on Jetty. However in most cases the site doesn't need to be served via a web server but can be used directly from the filesystem with a browser.

This mojo has the following disadvantages:

- dependency footprint is huge
- several CVEs reported in used Jetty dependencies (#1274)

Update:
Currently there is on demand-rendering (hot-reload) implemented via https://github.com/apache/maven-site-plugin/blob/master/src/main/java/org/apache/maven/plugins/site/run/DoxiaFilter.java. Maybe instead implementing a hot-reload just based on filesystem monitoring should be used.

Start by reading src/main/java/org/apache/maven/plugins/site/run/SiteRunMojo.java and DoxiaFilter.java to understand the current Jetty-backed site:run and on-demand rendering. Investigate the plugin dependencies and the filesystem-monitoring alternative described in the update; done means the unnecessary Jetty dependency footprint is removed while the intended site-preview behavior is addressed.

Written by the indexing model from the issue text.