Update Node and Yarn dependency + install guide

[ekansa]

brief description of changes

Implement similar change to master branch for PR already accepted for 7.3 branch, in response to this issue: (https://github.com/archesproject/arches-docs/issues/275)

issues addressed

(https://github.com/archesproject/arches-docs/issues/275)

further comments

The edit here saw extensive discussion and reflects the current consensus about how to best explain navigating Node.js and Yarn dependencies. It also has a minor clarification on PostresSQL version expectations.

Should a similar update be made for the v6.2 documentation? I'm less familiar with v6x requirements.

This box must be checked

• [X] the PR branch was originally made from the base branch

This box should be checked

• [X] after these changes the docs build locally without error

This box should only be checked you intend to follow through on it (we can do it on our end too)

• [ ] I will cherry-pick all commits in this PR into other branches that should have them after this PR is merged

[mradamcox]

re: v6.2, if it's easy it can't hurt, but I would think that having this update in both /stable and /latest builds will get it in front of the right people.

[ekansa]

@mradamcox Perfect! I'll make the changes to the v6.2 branch. Thanks for the review and suggestions!