Update "Getting Started" Page to outline PHP installation process

[joehoyle]

Description

Our Getting Started guide starts essentially with "Install Composer", but it doesn't even mentioned that you'll need PHP installed, or what version of PHP. In https://github.com/humanmade/altis-dev-tools-command/pull/34 we had one reported case where someone is using PHP 7.2 for example which doesn't since we switched to PHP 7.4+. It seems currently we don't acknowledge or guide people on installing PHP 7.4 which is a prerequisite to composer, which is a prerequisite to altis.

Acceptance Criteria

• [ ] Documentation explains how to install PHP for Mac, Linux and Windows

Repo - https://github.com/humanmade/altis-documentation/tree/master/other-docs/getting-started

Ready

• [x] Is the title clear?
• [x] Is the description clear and detailed enough?
• [x] Are acceptance criteria listed?
• [x] Have any (potential) dependencies been identified?
• [x] Is an estimate/timebox assigned?
• [x] Is a priority label assigned?
• [x] Is this ticket added to an epic? (Optional)

[robindevitt]

Along with mentioning that they need to have PHP installed, we would probably need to give them direction to installing PHP. Same as what we have done for composer.

Homebrew - https://brew.sh/

[wisyhambolu]

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