Update the current gate 'slides' to build and publish upstream
training content. This patch does the following changes.
* Adds generateslides script to use sphinx/hieroglyph to build slides
* Updates tox.ini to use generateslides script
* Update .gitignore to ignore _build/ directory which is the default
folder for generated output generated by sphix.
Hence forth, any changes to the upstream training rst slides will be
gated and published, also it is essential to note that these jobs are
new and may not be accurate or reliable to address all the errors in
the rst slides.
Change-Id: I8a220dd31501f9b3733a6282d7048fe283d2a0d1
9ac3590de7