[bitcoin/bitcoin] Docs: Create default bitcoin.conf file on startup (#13761)

Yeah, currently the test framework always puts the `datadir`s in a custom path deep in the system `/tmp` directory. This prevents polluting the $HOME path of the machine running the tests. But because of this, my change is basically untestable, unless we change the way the test framework works or hack something ugly.

Testing default datadir creation could be a testing feature we add in a later PR.