Merge remote-tracking branch 'remotes/pmaydell/tags/pull-docs-20200312' into staging

Merge remote-tracking branch 'remotes/pmaydell/tags/pull-docs-20200312' into staging

docs queue:
 * Remove some no longer needed texinfo infrastructure
 * Reorder the top level index docs to put most useful manuals first
 * Split the Arm target-specific info into sub-pages
 * Improve the Arm documentation a bit with info previously
   only on the wiki page

# gpg: Signature made Thu 12 Mar 2020 11:42:10 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-docs-20200312:
  docs: Be consistent about capitalization of 'Arm'
  docs: Move arm-cpu-features.rst into the system manual
  docs/system/target-arm.rst: Add some introductory text
  docs/system: Split target-arm.rst into sub-documents
  Makefile: Allow for subdirectories in Sphinx manual dependencies
  docs/qemu-option-trace.rst.inc: Remove redundant comment
  docs/index.rst, docs/index.html.in: Reorder manuals
  Makefile: Make all Sphinx documentation depend on the extensions
  docs/sphinx/hxtool.py: Remove STEXI/ETEXI support
  hxtool: Remove Texinfo generation support
  Update comments in .hx files that mention Texinfo
  Makefile: Remove redundant Texinfo related code

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