docs: add theme-color meta tag

[McAllaster]

Add a theme-color meta tag with Hono's orange branding.

I personally think the docs site looks slick with the orange accent on compatible devices, but feel free to reject this if you feel otherwise.

[yusukebe]

Hi @McAllaster

Interesting! But I don't know if we should merge.

On my iPhone, I turned on the dark mode on iOS and the webpage dark mode, which currently looks like the following:

I applied this change:

I think the orange is incompatible.

But it's a good idea. I want to hear others' opinions.

[McAllaster]

Yeah, Hono's orange is definitely a bold and fiery color! If you're not sure about it, I'm sure other folks feel the same. Keeping it as-is is definitely the safer choice, but I figured I'd at least put this out for discussion.