issue #5082

API is not finished.

issue #5082

this commit breaks the apidoc extension. will adjust it afterwards.

New syntax:

```
[link to guide](guide:file-name.md)
[link to guide](guide:file-name.md#subsection)
```

fixes #4719

issue #746

added class references (current version without hyperlink)

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

```
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
```

separated guide and apidoc, better maintainability

fixes #2220

- supports GFM now
- allows defining different flavors

fixes #47