Add guide on how to update a local Open SaaS app

[vincanger]

Description

Fixes #240

Contributor Checklist

Make sure to do the following steps if they are applicable to your PR:

• [ ] Update e2e tests: If you changed the /template/app, then make sure to do any neccessary updates to /template/e2e-tests also.
• [ ] Update demo app: If you changed the /template/app, then make sure to do any neccessary updates to /opensaas-sh/app_diff also. Check /opensaas-sh/README.md for details.
• [x] Update docs: If needed, update the /opensaas-sh/blog/src/content/docs.

[vincanger]

ok @Martinsos I made the changes and leaned more heavily on advising against updating.

[vincanger]

ok @Martinsos -- one more review please :)

[vincanger]

Ok I simplified it even further based on your comment and am merging the changes @Martinsos -- good ideas.