Add OpenSearch support

[maximilian-walter]

Describe the feature you'd like

Add XML-file and HTML meta-tag so browsers can autodiscover the search of BookStack instances. See:

https://developer.mozilla.org/en-US/docs/Web/OpenSearch

Describe the benefits this would bring to existing BookStack users

Easily add a BookStack instance as search-engine to browsers supporting this standard (like Firefox)

Can the goal of this request already be achieved via other means?

Not easily, Firefox for example has no UI to manually add search engines, as far as I know.

Have you searched for an existing open/closed issue?

• [X] I have searched for existing issues and none cover my fundamental request

How long have you been using BookStack?

1 to 5 years

Additional context

I can prepare a pull request if you accept this new feature.