Add content fields to ArticleViewDocument

[Prokyonn]

Q	A
Bug fix?	no
New feature?	no
BC breaks?	no
Deprecations?	no
Fixed tickets	fixes #issuenum
Related issues/PRs	#issuenum
License	MIT

What's in this PR?

Adds an additional contentField to the ArticleViewDocument.
The contentField holds all properties, which are tagged with sulu.search.field in the xml configuration.

Thus the developer can easily index custom fields and use them e.g. in a SearchController.

Why?

Some projects need more data in the index to create a desired search behaviour.

Example Usage

        <property name="text" type="text_editor" mandatory="true">
            <meta>
                <title lang="en">Text</title>
                <title lang="de">Text</title>
            </meta>

            <tag name="sulu.search.field"/>
        </property>

To Do

• Add tests
• Add documentation page

[wachterjohannes]

@Prokyonn can you add a documentation section about this tag?