Initial doc about working with Gradle (#10387)

* docs: Initial doc about working with Gradle

* docs: Document configurations

* docs: Document dd-trace-java's useful extensions

* docs: Add troubleshooting section

* docs: Add section on tasks

* docs: Tweak troubleshooting

* docs: Mentions plugins block

* chore: typos

* chore: Add a section on dependencies

* chore: Mentions dependency locking

* chore: Various wording changes

* chore: Quick reference

Author: bric3