Unify developer environment setup and documentation

Motivation

Currently, the docker-compose setup is (somewhat) explained in the following places:

• https://github.com/rucio/documentation/blob/4f8b1d959b3c978f40419cbb940656d146f07008/docs/operator/setting_up_demo.md (I would argue this belongs in the developers section, not operators)
• https://github.com/rucio/rucio/blob/c7cab8a5bb69a579da6aa4104c88bca7ebb4f2e6/etc/docker/dev/README.rst (the source document for the above documentation)
• pieces of information in Setting up a Rucio Developer environment … pages for developers (can stay there)
• https://github.com/rucio/documentation/blob/4f8b1d959b3c978f40419cbb940656d146f07008/docs/developer/testing/fts_and_conveyor.md
• deprecated information in https://github.com/rucio/documentation/blob/4f8b1d959b3c978f40419cbb940656d146f07008/docs/developer/webui/webui_frontend_vscode_dev_env.md#connecting-to-a-rucio-instance

I wanted to update and extend the existing documentation, but I am not sure where.

Proposed changes

I would like to consolidate the information, removing the old readme file from the main repository and moving it entirely to the developers section of the documentation website. I will be waiting on a go from the documentation component leads, though.