docs: update LSP quickstart (#28954)
The LSP quickstart can act as our true "entrypoint" for answering the
question "How do I use LSP in Neovim?" As such, it can be a little more
beginniner-friendly than other sections of our help docs by including
explanatory comments and a more fleshed out example (including a
`FileType` autocommand).
This also includes some other minor wording updates and points users
toward `:checkhealth lsp`.
(cherry picked from commit 28c04948a1)
Co-authored-by: Gregory Anders <8965202+gpanders@users.noreply.github.com>
777e15fa61
