Improve header documentation

[gfellerph]

The header docs does not have any controls (args). Add the following args to the header docs.

Controls

• Application title: [boolean and string] Controls the presence and content of the slotted <h1>
• Meta navigation: [boolean] Controls the presence of the meta navigation ("About us" and "Jobs")
• Custom controls: [boolean] Controls the presence of default slot content (currently "Search" and "Login" links in the local header)

Slots

• post-logo: Should be used together with the <post-logo> component (Remove default slot on host for post-logo #4276)
• meta-navigation: Holds an ul with meta nav links
• post-togglebutton: Holds the mobile menu toggler (Remove default slot on host for <post-togglebutton> #4277)
• post-language-switch: For the post-language-switch component
• title: Application title
• default: Custom controls or content, right aligned in the local header
• post-mainnavigation: Has a default slot because it's only meant to be used in the <post-header>

Required CSS imports

• header
• list
• button

Utility classes should not be necessary to style the header (#4278).

Tasks

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1. Add the necessary controls
2. Document the slots
3. Add documentation about what files are necessary to import