Unified documentation

[mondalaci]

Make the existing "Smart macro reference" tab appear on all pages and rename it to Documentation.

At the top of the documentation page, create a sticky "Table of contents" link that should display a menu when hovered. Create menu items and link them according to the following list:

• Reference manual - copy the markdown content of https://github.com/UltimateHackingKeyboard/firmware/blob/master/doc-dev/reference-manual.md as HTML.
• User guide - copy the markdown content of https://github.com/UltimateHackingKeyboard/firmware/blob/master/doc-dev/user-guide.md as HTML.
• Interactive manual - Feature the actual content of the Smart macro reference sidebar here.
• Accented characters - copy the content of https://ultimatehackingkeyboard.com/blog/2018/06/23/how-can-i-type-accented-characters-with-my-uhk post to this page.
• Custom scancodes - copy the content of the https://ultimatehackingkeyboard.com/blog/2021/10/31/production-progress-spare-parts-bonanza-new-agent-release page starting from "The first feature is arbitrary scancodes" and ending with "applications recognize different sets of scancodes."
• Agent shortcuts - Copy the content of the current Help page and remove the old page.

Don't use a webserver like we used to, but create static HTML pages within Agent.

Retrieve the firmware markdown pages from the bundled firmware release.