Update the Getting Started tutorial

[matt-bernhardt]

While working with Dave on getting the network running locally during his onboarding, we realized that the Getting Started article did not address some things, and was slightly out of date.

This expands the Getting Started tutorial slightly, adding information about Composer authentication and a slightly different order of steps.

There are some additional updates as well, dealing with installation methods and PHP versions.

Developer

Stylesheets

• Any theme or plugin whose stylesheets have changed has had its version
  string incremented.

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• All new secrets have been added to Pantheon tiers
• Relevant secrets have been updated in Github Actions
• All new secrets documented in README
• No secrets are affected by this change

Documentation

• Project documentation has been updated
• No documentation changes are needed

Accessibility

• ANDI or Wave has been run in accordance to
  our guide and
  all issues introduced by these changes have been resolved or opened as new
  issues (link to those issues in the Pull Request details above)
• There are no accessibility implications to this change

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• Stakeholder approval has been confirmed
• Stakeholder approval is not needed

Dependencies

NO dependencies are updated

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• The commit message is clear and follows our guidelines
  (not just this pull request message)
• The changes have been verified
• The documentation has been updated or is unnecessary
• New dependencies are appropriate or there were no changes

[djanelle-mit]

Now that my local is running...

I think this captures all of the steps we did originally. I'm not sure where my database/files issues arose, but until we figure that out I think this feels clear to me.