Fix search focus when s/f is pressed

On the Modules matrix page the Search input steals focus when the Module filter input is active and s/f is pressed. This makes it impossible to search for modules containing "s" or "f". It is a known problem in mkdocs-material - it was fixed, however the fix only works for inputs that are within a <form /> (https://github.com/squidfunk/mkdocs-material/issues/398).

This commit wraps the input in a form so that the focus stays in the filter when s/f is pressed.