Update docs

[dentalfloss1]

Clarify build instructions in the README file for users who are not using Docker.

Add information on making new packet formats in the C++ development section of the docs.

[jaycedowell]

Some other things to think about:

• Should the Makefile in docs also run doxygen as part of building the docs?
• Should there be some toggle in configure/a Makefile that makes things work even without doxygen?
• Should we also test for sphinx/breathe or something like extras_require in the setup.py?
• Should the top-level Makefile have a make docs option that builds everything if Python/doxygen are installed?

[jaycedowell]

I've made some changes to configure. It now:

• emits a warning if doxygen isn't found about how the docs cannot be built
• tests for the sphinx-build, sphinx-apidoc, and breathe-apidoc scripts
• emits a warning if sphinx-* or breathe-apidoc are not found about how the Python docs cannot be built
• updates make doc to build all possible docs with the Python stuff built as "singlehtml"
• updates docs/Makefile to a .in file so that it is consistent with all other build files and uses the configure-detected values for the paths.

I think that covers it. We should also update master once we're happy with these doc build changes.