BUG: Staticman instructions need update

[VincentTam]

Bug Report

Describe the bug

Wiki's Staticman guide that I've written one and a half year ago needs to be rewritten due to Staticman's maintainer's recommendation to stay way from any public API instance in eduardoboucas/staticman#317 (comment).

Additional context

Staticman version	authentication	GitLab support?	POST URL
2	GitHub bot (legacy)	❌	/v2/entry/user/repo/property/
3	GitLab bot, GitHub Apps	✔️	/v3/entry/gitProvider/user/repo/property/

Thanks to the PR with one-click deploy button to Heroku, building a private API instance on Heroku requires only config of environment variables via the online dashboard. I'm going to edit this repo's wiki page on Staticman to provide an illustrated guide for the setup.

ℹ️ I welcome questions about the functional aspects of Staticman. However, my lack of knowledge in node.js might hinder me from answering your technical questions. For question about the server-side setup, it's better to ask in Staticman's official repo. For front-end setup (theme-specific), one may ask it in the theme's repo.

[VincentTam]

Done at https://github.com/pacollins/hugo-future-imperfect-slim/wiki/staticman.yml/8d7b1c6b70c90c41e754e78899b3978fe76a1661