Document environment variable used for Remote Development
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## Description
Context - https://gitlab.com/gitlab-org/gitlab/-/merge_requests/140458#note_1707570053
* Create tables of all Remote Dev ENV vars we use, similar to https://docs.gitlab.com/ee/administration/environment_variables.html
* The table for the user-facing `GITLAB_*` vars can live in user-facing docs, somewhere under https://docs.gitlab.com/ee/user/workspace/
* The table for internal `GL_*` vars can live in developer-facing docs
* For now, this can be in https://gitlab.com/gitlab-org/remote-development/gitlab-remote-development-docs/-/blob/main/README.md somewhere.
* Going forward, I think we should create our own section for `Remote Development` under https://docs.gitlab.com/ee/development/feature_development.html#gitlab-components-and-features. This can initially just point to https://gitlab.com/gitlab-org/remote-development/gitlab-remote-development-docs/-/blob/main/README.md, but we can eventually start migrating parts of that which make sense into the main docs repo.
See also https://gitlab.com/gitlab-org/gitlab/-/merge_requests/134984+, which involves refactoring and standardizing our handling of `GITLAB_*` ENV vars.
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