Document the code styling

[matthewdunsdon]

Feature request

As a contributor, I would like to know the code styling rules upfront, so that I am aware of them before before committing code.

I am using intellij, so maybe committing .idea/codeStyles would also be helpful.

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For completeness, the coding styles were referenced here, but have since been removed.
6456242#diff-8cc7b2b0d78dd2501610391c086a8516
By @sl-slaing in #1688 (comment)

[tjohnson-scottlogic]

Info from @afroggattsl:

The team previously just used the default IntelliJ styling rules, so committing them is the way to go. The developer guide should be updated to mention this.