docs: remove instructions for upgrading 2.3 to 2.4

Signed-off-by: Sebastian Kuzminsky <seb@highlab.com>
docs: add an intro blurb to Getting LinuxCNC

Signed-off-by: Sebastian Kuzminsky <seb@highlab.com>
docs: add an intro blurb to "Updating LinuxCNC"

Signed-off-by: Sebastian Kuzminsky <seb@highlab.com>