doc: apply sentence case to guides headers

PR-URL: https://github.com/nodejs/node/pull/37497


Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Pooja D P <Pooja.D.P@ibm.com>