webpage (tutorial/documentation/etc.) pipeline improvements

[Roger-luo]

This issue has been tagged for a bounty during #unitaryHACK

This issue is for tracking the web page related problems

Layouts

1. we need a more consistent solution to add banners to pages, see also (Documenter/#1132)
2. improve large screen experience (Documenter/#1563)
3. improve CSS to have a more consistent style.

Tutorial

1. our Pluto tutorial is included via <iframe> which has a delay and bad layout, unlike the previous tutorials based Literate.jl, we don't have an easy to download button at the moment
2. the tutorial page table of content (the left panel) has an inconsistent layout with the banner

Pipeline

ideally, we should move all documentation into one repo, and build them together, like DiffEq documentation, this requires

1. a CI that gets triggered (or a nightly cron job) for the master branch to rebuild new documentation parts for each component packages
2. integration with the front page

[Roger-luo]

@fonsp just show me a solution to insert iframe here: https://htmlview.glitch.me/ maybe we can reconsider using iframe for the tutorial part now

[VarLad]

Try implementing it with the existing tutorials to see how it goes?

[VarLad]

If anyone lands on this issue, kindly note that I've undertook this Hack