Update README.md
Description
We updated the documentation, but missed the README in the repo, which is similar to the docs main page before the revamp.
Expected behavior
The README should be up to date, e.g. not have an outdated "milestones" link, not call the Cargo ODD "Autonomous Depot Maneuvering", not have a broken "Functional feature roadmap" link etc.
The easiest way to stay up-to-date would be to deduplicate content with the source of truth, i.e. https://autowarefoundation.gitlab.io/autoware.auto/AutowareAuto/
Definition of Done
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Remove most of the content in README.md and simply link to the home page of the documentation