Ensure optimal page speed for docs pages

[stefanov-stefan]

Is your feature request related to a problem? Please describe.
For the purposes of SEO, page speed on mobile is one of the top ranking factors. In addition to that the page speed for desktop ensures optimal user experience. Hence, we should aim to achieve 90+ performance score on both for every article in the documentation, as measured by https://developers.google.com/speed/pagespeed/insights/ (there is also chrome extension)

Describe the solution you'd like
I would propose a few items:

1. Introduce a monitor for the page speed of the various product documentations (web teams have introduced such for the jqeury, mvc, core demos - ask Nencho Nenchev)
2. Introduce a build step, that will utilize a tool like TinyPNG which will convert the images to png upon upload, hence reduce their size, while keeping their qualiry
3. Introduce a built step, that will either result in error or warning when a docs article with sub-optimal page speed is introduced
4. Introduce an image guide document that describes the best practices for adding images to docs (consult with Svetlin Nikolaev)