Add docs linting to the Eclipse editor extension

As part of checking our repos for readiness to upgrade markdownlint-cli2 in gitlab-org/gitlab#562232 (closed), I didn't find a .tool-versions file in the gitlab-eclipse-plugin extension.

https://gitlab.com/gitlab-org/editor-extensions/gitlab-eclipse-plugin/ was created later than the other editor extensions. It was after the user docs were moved into the gitlab repo, so docs linting was missed.

This issue tracks the decision of whether or not to implement docs linting in that project, and any MRs raised if we do implement it.

• @scawood will want to know about this issue because he handles TW duties for groupeditor extensions
• @eread will want to know as part of the tooling team