docs: added a new section in installation.md to get the mongodb connection string from cloud.mongodb.com

[kunstewi]

Issue:

fixes #3984

PR attempts to fix. --> just added a new section about getting the mongodb connection string from cloud.mongodb.com

Demo:

Changes:

just a new section added in installation.md file about getting the mongodb connection string from cloud.mongodb.com, in case new contributors have trouble setting up mongodb locally.

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• has no typecheck errors (npm run typecheck)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number, i.e. Fixes #123
• meets the standards outlined in the accessibility guidelines

@raclim

Thanks for your time.

[raclim]

This makes sense to me so far, thanks so much!