Add a How-To doc for overriding headers and footers in an MFE

[davidjoy]

I was trying to track down our documentation on doing MFE header/footer overrides, and found that we seem to have lost it in the transition to docs.openedx.org.

This commit in particular deleted a page in edx-developer-docs that listed out environment variables, how to do branding, how to override headers/footers, etc:

openedx-unsupported/edx-developer-docs@a4d425d

It replaced the page with a link to frontend-platform's documentation, which is merely doc for the frontend-platform library, not for our "Frontend Platform" in general. That JSDoc site isn't really a good place for the info we lost.

I'm happy to work on this task, but would love some input as to where we'd like this info to live now.

[davidjoy]

Tagging @feanil just in case you have any more context you'd like to add on the removal (maybe I've just lost where it ended up!)

[feanil]

@davidjoy thanks for catching this. I think the missing seems like it would best fit as a new How-To in the developer section, something like "How to override the header/footer in an MFE".

You can use the template for a new how-to which can be found here.