Feature: Implement GoDoc under cursor for filetype "godoc"

[tylerFowler]

What did you do? (required: The issue will be closed when not provided)

Opened :GoDoc pkg.Struct via command mode shortcut "K", resulted in a new pane with filetype godoc, and then found another symbol I wanted to get docs about.

What did you expect wish to happen?

Tried the "K" shortcut again on that symbol, in the "godoc" pane hoping to get the godoc for that symbol for navigation.

What happened instead?

:!man 'InterestingSymbol'
[No write since last change]
No manual entry for InterestingSymbol

shell returned 1  

Then had to manually type & run :GoDoc pkg.InterestingSymbol

Configuration (MUST fill this out):

Fairly vanilla, nothing novel here

vim-go version:

Commit 3d16fa33a9a1b707a15da3ca4e3f2a7374d0902a

vimrc you used to reproduce:

Not relevant - this is a feature request

Vim version (first three lines from :version):

VIM - Vi IMproved 9.0 (2022 Jun 28, compiled Feb  7 2023 12:17:42)
macOS version - x86_64
Included patches: 1-1276

Go version (go version):

go version go1.19.6 darwin/amd64

Go environment

Not particularly relevant, but using Go modules not GOPATH.

[tylerFowler]

Awesome thanks a bunch for the quick implementation! This cuts out a lot of small bits of toil for me.