added class references (current version without hyperlink)

- better title page,
- book style,
- improved internal linking using labels and references

fixes #2797

fixes #3539

```
vendor/bin/indent --spaces -r framework/
vendor/bin/indent --tabs -r . --pattern=*.json
vendor/bin/indent --spaces -r . --pattern=*.md
vendor/bin/indent --spaces -r . --pattern=yii.*.js
vendor/bin/indent --spaces -r . --pattern=*.css
```

fixes #3551

issue #3494

without check

fixes #3438

breaking changes.

This change fixes the issue #3292. The details were mentioned in the comments to the issue.

To comply with the docs RE namespace usage:
https://github.com/yiisoft/yii2/blob/master/docs/guide/extend-creating-extensions.md