docs(www): add install instructions component

[timdeschryver]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ngrx/platform/blob/main/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Each install page has instructions for npm and yarn.

What is the new behavior?

This PR introduces a instructions component to document the installation script.
It also adds pnpm because I think it's used a lot more frequent than yarn these days.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

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