Update boilerplate link from deprecated repo to latest

[ixje]

While going through the docs I realized that the docs appear to be pointing to the repo -deprecated-app-boilerplate while there is a non-deprecated version. I could not get the former to work while I could get the latter to work. This updates the documentation.

[cfranceschi-ledger]

Comment added just before repo being archived : The changes proposed in this PR have been integrated to the new documentation (see developer-portal)