Add FE selectors docs

What does this MR do?

Rename Design Patterns docs to Best Practices.

Add FE docs for best practises when creating/using selectors.

From discussion at https://gitlab.slack.com/archives/C0GQHHPGW/p1553097051478400

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer