improve docs for Gentoo Linux installation

Changes summary:
  * Added available USE flags table(description included).
  * Added some tips regarding the use of docker in the Gentoo environment.
  * Sorted out docker installation ways.
  * Added information about the newly created Gentoo Docker team.
  * Misc improvements

Signed-off-by: Pavlos Ratis <dastergon@gentoo.org>
Ditto.