Add content philosophy

[JackMorganNZ]

Description

Create content philosophy for documentation, which will be created from a combination of:

• Existing core principles listed on website: http://csunplugged.org/principles/
• Extra comments by @tah942 and @timbellnz

This also includes providing templates for unit plans, lessons, follow up activities, programming exercises, etc, and also providing details for chosen structure of topic > unit plan > lesson, etc.

The documentation file is located at cs-unplugged/docs/source/author/writing_philosophy.rst but should be changed to cs-unplugged/docs/source/author/content_philosophy.rst.

Provides extra content for #29.

Checklist

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