docs: use a bold motivation label for the reason line in module docs #1152

follow up from #1150, the reason line under the title of each doc should be prefixed with a bold label so it reads clearly:

Motivation: cpx hasn't had a release since 2016...

this would update the doc template, the guideline in contributing, and the docs that already have a reason line (better-auth-cli, rollup-plugin-terser, tsup, cpx)

should the docs that already open with a reason (e.g. crypto-js, uri-js, bcrypt) get the label as well? i think yes so every doc reads the same