docs: add plugin section

[kid1412621]

since wiki is not fully documented so this is useful for newcomers

[andrewflbarnes]

Don't think this belongs under the workflow section - instead would be better to have a new section 2 plugins (which seem more important than common workflows), and renumber the common workflows section(s) to 3(...).

Also, given how sparse the information is probably nicer as a table? e.g.

2 Plugins

The below plugins can be enabled with jenv enable-plugin <plugin> and disabled with jenv disable-plugin <plugin>

Plugin	Effect
export	Set and export JAVA_HOME to the jenv version on every cd
gradle	Set and export GRADLE_OPTS to the jenv options when running gradle
maven	Set and export MAVEN_OPTS to the jenv options when running maven
...

3 Common Workflows

...