Previously API reference is gigantic one rst file and it is a bit hard
to use, especially when browsing similar functions.  This commit
splits API reference into smaller fine grained files.  The macros,
enums, types are now in its own file.  Each API function has its own
file now.  API reference doc is now index to above documentation
files.  The apiref-header.rst is renamed as programmers-guide.rst and
becomes standalone document.

- Tell Vim this is a Dockerfile to enable syntax highlight;
- Explicity use Ubuntu "Trusty";
- Remove downloaded file to save space;
- Chain up some RUN commands to generate fewer layers.

fix the typo and add the missing command in sample command

--tls-ctx-per-worker option does not work well of OCSP stapling.  Also
it makes session ID useless.

The reverted part of the change may use not connected socket in HTTP/1
backend.

on_SOMETHING should be used only for I/O callback functions