Remove some metadata / yaml from docs

Nobody has claimed this yet.

I was wondering if we actually need the metadata defined in the beginning of `docs/*.md` files?

```
category: Develop
previous: tests-ui
next: tests-travis
subGuides:
- plugin-settings
- piwiks-ini-configuration
```

In theory this information is already present in the category classes, isn't it? For example here: https://github.com/piwik/developer-documentation/blob/master/app/helpers/Content/Category/DevelopCategory.php

Depending on the path or filename of the markdown file we know the `category`. Currently, when moving a page to another category I do have to update the PHP category classes and update the related markdown file.

Also `previous` and `next` could be maybe automatically generated for all pages and not needed to be defined in the markdown files since we have that information already in the category class? Meaning we can find out whether there is a previous or next sibling.

With `subGuides` I was confused why it is sometimes defined in the markdown file and sometimes in a category class. I think we could just remove it and for simplicity always define it in the category class so one can immediately see the whole structure there and change things in one place.

Of course we'd have to create all category classes to know all this information but that should be fast. Especially since we cache pages.

Compare the metadata in docs/*.md with the category definitions in app/helpers/Content/Category/DevelopCategory.php and the other category classes. Determine whether category, previous/next, and subGuides can be represented consistently in the category classes; done means the duplicated front matter is removed without losing the documented structure or navigation.

Written by the indexing model from the issue text.