doc: apply sentence case to headers in doc/guides

PR-URL: https://github.com/nodejs/node/pull/37506


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Pooja D P <Pooja.D.P@ibm.com>
Reviewed-By: Harshitha K P <harshitha014@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>