Merge branch 'cs-frontend-guidelines' into 'master'

Add Frontend Development Guidelines to the Developer Documentation

## What does this MR do?

WIP incomplete draft of some Frontend Development Guidelines for contributors to the project, new hires, and for other projects and persons to look to for guidance.

These are all subject to removal, modification, etc. If you disagree with any of these, please feel free to voice your concerns :)

## Why was this MR needed?

There are a number of guidelines we currently follow implicitly, this makes them more official and available for reference/discussion.

It also exists as a reference for certain patterns we use which may be unfamiliar to new contributors, e.g. the per-page JS.

Feel free to give feedback on wording, missing sections/guidelines, whether a guideline should even exist, etc.

See merge request !5477

doc/development/README.md

@@ -13,6 +13,7 @@
 - [SQL Migration Style Guide](migration_style_guide.md) for creating safe SQL migrations
 - [Testing standards and style guidelines](testing.md)
 - [UI guide](ui_guide.md) for building GitLab with existing CSS styles and elements
+- [Frontend guidelines](frontend.md)
 - [SQL guidelines](sql.md) for SQL guidelines
 
 ## Process