[Eclipse] Document GitLab for Eclipse configuration

Problem to solve

We need to document GitLab for Eclipse as we start to distribute it.

Proposal

Add user installation, usage, etc. documentation under https://docs.gitlab.com/ee/editor_extensions/eclipse/

Further details

Links / references

cloned from #25 (closed)

added Category:Editor Extensions Editor ExtensionsEclipse devopscreate groupeditor extensions sectiondev labels

added to epic &140

changed title from [Eclipse] Document GitLab for Eclipse installation to [Eclipse] Document GitLab for Eclipse configuration

added documentation maintenanceusability labels

added typemaintenance label

added Technical Writing UI text twtriaged labels

mentioned in commit gitlab-org/gitlab@15757688

mentioned in merge request gitlab-org/gitlab!175586 (merged)

mentioned in commit gitlab-org/gitlab@54055984

mentioned in commit gitlab-org/gitlab@08beea77

mentioned in commit gitlab-org/gitlab@1870a95a