Dev Portal: Custom headers/footers

## Jobs to be done (optional)
As a Dev Portal admin, I want a fully customized header and footer, so that I can better align my portal with my company's brand guidelines.

**Explanation**

Cosmetic changes to the Dev Portal header and footer currently requires extensive custom CSS. However, it is impossible to guarantee forwards compatibility with CSS. To solve this, it should be possible to create custom headers and footers that have no risk of being broken by general UI changes.

## Definition of done
Update docs

## Information
Aha: https://konghq.aha.io/epics/DEVP-E-138
PRD: https://docs.google.com/document/d/17_8Ji9Qb1jPHG4uaB7d4mEyfypkLAb3Zwo1qa8F4exQ/edit?tab=t.0#heading=h.yl3chg3ifuw

## Due date (optional)

## Size

Start by reviewing the linked Aha epic and PRD to understand the custom header and footer capability. Update the relevant Dev Portal documentation; done means the documentation covers the requested customization without requiring custom CSS.

Written by the indexing model from the issue text.