Enable building and installing rST docs with Sphinx

# gpg: Signature made Thu 07 Mar 2019 15:05:12 GMT
# gpg:                using RSA key E1A5C593CD419DE28E8315CF3C2525ED14360CDE
# gpg:                issuer "peter.maydell@linaro.org"
# gpg: Good signature from "Peter Maydell <peter.maydell@linaro.org>" [ultimate]
# gpg:                 aka "Peter Maydell <pmaydell@gmail.com>" [ultimate]
# gpg:                 aka "Peter Maydell <pmaydell@chiark.greenend.org.uk>" [ultimate]
# Primary key fingerprint: E1A5 C593 CD41 9DE2 8E83  15CF 3C25 25ED 1436 0CDE

* remotes/pmaydell/tags/pull-sphinx-20190307:
  MAINTAINERS: Add entry for Sphinx documentation infrastructure
  docs/conf.py: Don't hard-code QEMU version
  Makefile: Abstract out "identify the pkgversion" code
  Makefile, configure: Support building rST documentation
  docs: Provide separate conf.py for each manual we want
  docs/conf.py: Disable option warnings
  docs/conf.py: Don't include rST sources in HTML build
  docs/conf.py: Configure the 'alabaster' theme
  docs/conf.py: Disable unused _static directory
  docs: Commit initial files from sphinx-quickstart
  docs: Convert memory.txt to rst format
  docs/cpu-hotplug.rst: Fix rST markup issues

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>