doc: standardize on make -j8

Most of the docs recommend `make -j8`. The one exception is the pull
request template which recommends `make -j4`. This changes the pull
request template so that it is in line with the other docs.

PR-URL: #8678
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>

.github/PULL_REQUEST_TEMPLATE.md

@@ -9,7 +9,7 @@ Contributors guide: https://github.com/nodejs/node/blob/master/CONTRIBUTING.md
 ##### Checklist
 <!-- Remove items that do not apply. For completed items, change [ ] to [x]. -->
 
-- [ ] `make -j4 test` (UNIX), or `vcbuild test nosign` (Windows) passes
+- [ ] `make -j8 test` (UNIX), or `vcbuild test nosign` (Windows) passes
 - [ ] tests and/or benchmarks are included
 - [ ] documentation is changed or added
 - [ ] commit message follows commit guidelines