Convert README, INSTALL, etc. to Markdown

All documentation currently is being converted to Markdown. This task is to convert all .rst files to .md. See also issue for User Guide update: https://github.com/robotframework/robotframework/issues/5338

Files need conversion to .md.

• atest/README.rst

• atest/testdata/output/listener_interface/body_items_v3/README.rst

• atest/testdata/standard_libraries/telnet/README.rst

• doc/schema/README.rst

• utest/README.rst

• AUTHORS.rst we probably can remove in RF 7.6. It hasn't been maintained for ages and we can restore it from the history later if needed.

ToDo:

Include all .rst files on the project root as well.

Notes:

• See folder: https://robotframework.slack.com/archives/C063Y9GEMUP/p1790000646954529?thread_ts=1789980712.838279&cid=C063Y9GEMUP
• Please create an issue! List the files to be converted there as a task list so we can easily keep track of progress. One file per PR and let's start from filed not on the project root i.e. not part of the source distribution.