Fix documentation hierarchy

[eyeinsky]

From this issue #1443

(Added also the nix section header at the bottom so the existing paragraph would be easier to sight.)

[eyeinsky]

Looks like this now:

[alpmestan]

Perfect, thanks!

[akhesaCaro]

To support older version of GHC you need to explicitly import the <> operator.

import Data.Semigroup (<>)

You can also test it using the nix-shell (nix/nix-shell) and provide the version of GHC you want to use. See https://github.com/haskell-servant/servant/tree/master/nix

[eyeinsky]

Found a typo. (And though I pushed to my fork of the repo, github appears to have picked up the commit here too.)

[akhesaCaro]

This PR needs to be rebase. The CI will not build it in GHC < 8.6.5 anymore.

[gdeest]

Hmmm, I tried rebasing the branch but accidentally pushed current master on your branch instead of my rebase and GitHub seems to have interpreted this as a request to close the PR — not sure why but I can't push on your remote anymore.

Reopening the PR with my branch (sorry about this).

[gdeest]

#1465