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>
Merge pull request #8210 from dastergon/improve_gentoodoc

improve docs for Gentoo Linux installation