docs: fix ZCP dark mode and sidebar, embed coding agents video, add G…

[Vanshika-Rana]

• Minor Fix
• Minor Improvement

Bundles a few small fixes and one new docs section that came up in recent feedback:

• Fixed dark mode rendering issues on ZCP pages (Overview, How it works, Build and ship, Reference, Workflows in depth) — reported by Michal
• Fixed the ZCP sidebar submenu staying open when navigating from Reference back to Overview — also Michal
• Embedded the coding agents walkthrough video on the coding-agents page, per Aleš's request
• Added an "Advanced: Full Lifecycle Automation" section to the GitHub integration docs, referencing the new zerops-showcase-deploy reference repo (cleaned up and published under zeropsio as a prerequisite)

[FrancescoXX]

One quick comment: the link in the Overview that has the text "What remote workspace gives you" is redirecting to https://docsstage1-28c.prg1.zerops.app/zcp/setup/hosted-workspace#make-customization-persistent.

It shouls instead redirect to https://docsstage1-28c.prg1.zerops.app/zcp/setup/hosted-workspace