docs: add v2 docs links

[babycourageous]

Since folks are still on v2 and asking about those docs this adds a couple front-facing spots to find them:

1. main repo README
2. main RQ docs nav

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[TkDodo]

Awesome. Minor thing: it doesn’t look so good on mobile, and it’s hard to know on which docs I am because v3 is not “highlighted”:

[babycourageous]

Ah - you're totally right. My initial preview wasn't small enough to catch that.

I like what Tailwind does - but would be more involved, hehe

What do you think of keeping NAV as originally was - but putting a line on the homepage (like I did for README) that basically says "Still on v2? Check out the v2 docs here: LINK"

[TkDodo]

Yep, that sounds good

[babycourageous]

B4 pushing - how's this?

sm:

md:

lg:

[TkDodo]

I'd put the link under here because now it says v2 3 times in the same sentence 😅 . Or even like this:

Still using v2? No problem! Find the v2 docs here.

[babycourageous]

Went with the "Still using v2? No problem! Find the v2 docs here." option. Also - apologies for the accidental period in my commit msg. Didn't mean to do that.

[tannerlinsley]

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