Update Where Are My Certs section. (#3419)
Update Where Are My Certs section. (#3419)

* Update Where Are My Certs section.

This combines the `cert.pem` and `chain.pem` sections into a single paragraph,
making it clearer that they are closely connected. It also adds text indicating
that they are less common and moves them below the section for `fullchain.pem`.

* Update "Getting Help" section.

* Add link to document missing intermediate.

* Remove incorrect line about ordering.

Also remove "(as the filename suggests)," and clarify file ordering in the
fullchain.pem section.

stweil/letsencrypt,jsha/letsencrypt,lmcro/letsencrypt,jsha/letsencrypt,letsencrypt/letsencrypt,bsmr-misc-forks/letsencrypt,lmcro/letsencrypt,bsmr-misc-forks/letsencrypt,stweil/letsencrypt,letsencrypt/letsencrypt