fix(install-guide): fix links to chapters
chore(engine): business key unique constraint removal

- document removal and how to add it afterwards to your database schema

related to CAM-849
Merge branch 'develop' of github.com:camunda/docs.camunda.org

Conflicts:
	site/src/documents/guides/installation-guide/index.html.eco
	site/src/documents/guides/user-guide/toc.md
chore(db): Add additional subsection to toc