Fix Mobile Navbar for Collapse/expand nav sidebar

[Ayush41]

This PR resolves issue #11083.
Instead of adding the < > style button, the navigation sidebar was fixed by implementing the existing hamburger menu toggle to open and close the sidebar.

Changes Made

• Enabled hamburger menu toggle for opening/closing the sidebar
• Fixed the nav-hamburger menu
• Improved responsiveness of the navbar

FIXED DEMO

godot-fixednav_0na5QfsH.mp4

WITH ISSUE

godot-NavIssue.mp4

[Calinou]

Tested locally on desktop + mobile, there are some issues:

Notice the title overflowing, and the hamburger button not being in the top-left corner. (I've made sure to clear cache.)

Also, I'm not fond of fixed navbars myself, unless they disappear when you start scrolling down (and reappear when you scroll up). Fixed navbars limit vertical screen real estate quite a bit and makes the docs harder to read as a result.

Lastly, I don't think this PR addresses #11083 since it was specifically asking about a desktop use case. The main page contents still look squished if your browser width is just wide enough to make the sidebar show:

[Ayush41]

I'll look into it, in my system it's working fine and responsive. I'll update and resolve the actual issue mentioned :)