Better document Sphinx install instructions

The Sphinx build chain is fragile: any upgrades can cause it to break.

That's what happened this week, when I upgraded my machines to Fedora 26.

So, add a separate chapter at sphinx.rst with Sphinx install instructions, moving it out of changes.rst. and better name patch 2/6.

v2: improved the text at changes.rst to point to the file name with the Sphinx build instructions.

Mauro Carvalho Chehab (6):
changes.rst: Update Sphinx minimal requirements
docs-rst: move Sphinx install instructions to sphinx.rst sphinx.rst: explain the usage of virtual environment sphinx.rst: fix unknown reference
sphinx.rst: describe the install requirements for kfigure sphinx.rst: better organize the documentation about PDF build

 Documentation/doc-guide/sphinx.rst | 69 +++++++++++++++++++++++++++++++++++++-
 Documentation/process/changes.rst  | 16 ++-------
 2 files changed, 71 insertions(+), 14 deletions(-)

2.13.0