Move documentation to official website

We are currently considering giving the cratedb-toolkit more exposure in support processes to standardise the collection of certain information.

Right now, documentation is available at https://cratedb-toolkit.readthedocs.io/. https://cratedb.com/docs/guide/ has a link to it under the "TOOLS" section which links to readthedocs.io. This has the following disadvantages:

1. The design is not consistent across the standard cratedb.com/docs section and readthedocs.io
2. When sending users a link directly to the readthedocs.io documentation, there is no way back to the main documentation. It is disconnected.
3. ~The toolkit documentation may look unofficial, since it is hosted on a 3rd party domain that is not affiliated with cratedb.com~ (fixed in https://github.com/crate/infrastructure/pull/3461)

Documentation of other tools is directly integrated into cratedb.com/docs, for example crash, croud, or crate-python.