Cleanup E2E Best Practices documentation

What does this MR do?

• Rewrite API>UI section
• Add avoiding superfluous expectations section
• Add preference for Commit resource over ProjectPush section
• Change some wording in 'Ensuring tests do not leave the browser logged in' section

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content