Add a new doc for WordCamp Contributor Day

[danielbachhuber]

Fixes https://github.com/wp-cli/wp-cli/issues/5811

[danielbachhuber]

@schlessera @swissspidy Is there anything you'd add to or change about this doc?

[swissspidy]

A few thoughts at first glance:

• Maybe don't make it WCUS-specific? Or ist the idea to do make it more generalized after WCUS?
• Linking to https://make.wordpress.org/cli/handbook/contributions/contributing/ and https://make.wordpress.org/cli/handbook/contributions/pull-requests/#setting-up would make sense to avoid duplicate docs
• https://make.wordpress.org/cli/handbook/contributions/pull-requests/#setting-up needs some updates though, seems quite outdated and does not mention wp-cli-dev

[danielbachhuber]

• Maybe don't make it WCUS-specific? Or ist the idea to do make it more generalized after WCUS?

The idea is that we can replace "WCUS" with "WordCamp X" prior to each WordCamp. If there are multiple WordCamps on the same weekend, we can solve that problem when we get there.

• Linking to https://make.wordpress.org/cli/handbook/contributions/contributing/ and https://make.wordpress.org/cli/handbook/contributions/pull-requests/#setting-up would make sense to avoid duplicate docs

I intentionally copied what I thought was relevant 😊 This way, we have one doc to serve as a starting place (optimized happy path) instead of having the user get lost in the handbook.

• https://make.wordpress.org/cli/handbook/contributions/pull-requests/#setting-up needs some updates though, seems quite outdated and does not mention wp-cli-dev

Yeah. wp-cli-dev installation instructions could use some system requirements too, as well as direction on how to fork a repository to submit a pull request.