PRO-3105 - added --a2 flag to start project

[matthewhiggins415]

Summary

Closes issue PRO-3105. Added --a2 flag to the start project guide. Also included a quick note of which version of Node to have installed.

What are the specific steps to test this change?

For example:

1. Basically just added the flag and the node version note, so just make sure in the right spot. should be good.

What kind of change does this PR introduce?

(Check at least one)

• [ ] Bug fix
• [ ] New feature
• [ ] Refactor
• [x] Documentation
• [ ] Build-related changes
• [ ] Other

Make sure the PR fulfills these requirements:

• [ ] It includes a) the existing issue ID being resolved, b) a convincing reason for adding this feature, or c) a clear description of the bug it resolves
• [ ] The changelog is updated
• [x] Related documentation has been updated
• [ ] Related tests have been updated

If adding a new feature without an already open issue, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

[linear[bot]]

PRO-3105 The a2 documentation for creating a new project with the CLI is incorrect

The CLI tool has changed to require the `--a2` flag to create a new project using a2. The documentation for a2 does not reflect this. Installation of a2 sites also now requires at least the latest version of Node 12, and the documentation should reflect this.