List optimizations tricks and best practices

# Code size
- External function (and when it works)
- Using internal calls instead of modifiers
- How to split a large contract into smaller component
- ...

# Transaction gas cost
- Local variables instead of multiple storage read
- ...

Review the existing guidelines and training material to find where optimization guidance belongs. Cover the requested code-size and transaction-gas topics, including the listed examples, and consider the work done when the material gives complete, accurate best-practice guidance for those areas.

Written by the indexing model from the issue text.