Legacy 4.x links point to new docs

[lineus]

when using the legacy docs for 4.x, clicking a link in the guide should probably resolve to another 4.x doc instead of the current ones. Multiple pages in the 4.x docs have links with absolute paths to the current docs, ie. Clicking on the middleware link on the [4.x subdocs] (http://mongoosejs.com/docs/4.x/docs/subdocs.html) guide for instance, will take you to the current docs. I was going to update them and create a pull request but when I checked out the master branch I didn't see a 4.x folder under docs/ . Are the 4.x docs derived from the 4.x branch ? Just a subtle nudge in the general direction and I'll get it done :)
edit: I found the relevant info in the makefile for how the 4.x legacy docs get created.
is this a worthwhile endeavor?

[sobafuchs]

not sure, thoughts @vkarpov15 ?

[vkarpov15]

Thanks for pointing this out, I think I fixed most of them in 3660092, will deploy those changes with 4.13.11. Feel free to use those changes as an example if you find any others, and make a PR against the 4.x branch.