Documentation: not learning core git commands.

Documentation: not learning core git commands.

The initial section of tutorial was too heavy on internal
workings for the first-time readers, so rewrite the introductory
section of git(7) to start with "not learning core git commands"
section and refer them to README to grasp the basic concepts,
then Everyday to give overview with task/role oriented examples
for minimum set of commands, and finally the tutorial.

Also add to existing note in the tutorial that many too
technical descriptions can be skipped by a casual reader.

I initially started to review the tutorial, with the objective
of ripping out the detailed technical information altogether,
but I found that the level of details in the initial couple of
sections that talk about refs and the object database in a
hands-on fashion was about rigth, and left all of them there.  I
feel that reading about fsck-index and repack is too abstract
without being aware of these directories and files.

Signed-off-by: Junio C Hamano <junkio@cox.net>