docs: add version switcher to docs page

[paulbalandan]

Description
Implements a switch in the documentation page to allow us to switch between the (1) stable version hosted at https://codeigniter.com/user_guide/ and (2) latest (in-progress) hosted at https://codeigniter4.github.io/userguide/. If a local build, another option will also be displayed. The CSS theme uses the already supplied CSS by RTD.

On load:

When clicked:

I'm not sure if this should be in develop or on 4.8. Happy to change if needed.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[patel-vansh]

Since this is not any change to the framework code, I think develop is the right branch for this.

Just one thing that caught me was the text color of the selected item. As you can see in the On Load screenshot, the currently selected item is be kind of less visible than the above CodeIgniter logo. Idk how the theming works in the docs, but from a general viewer's perspective, it would be nicer if the Local build or any selected item's text color would be white.

Also, if possible, then maybe capitalize the first letter of each word?

[paulbalandan]

The CSS used here is actually the one shipped from sphinx_rtd_theme, like in their docs:

If others agree as well, I can make it a bit more noticeable.