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 <[email protected]>
Reviewed-By: Luigi Pinca <[email protected]>
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Johan Bergström <[email protected]>
Reviewed-By: Ilkka Myller <[email protected]>
Reviewed-By: Michael Dawson <[email protected]>
Reviewed-By: Daniel Bevenius <[email protected]>
Reviewed-By: Gibson Fahnestock <[email protected]>

Author: Trott

.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