Please keep in mind that pduerseler's way of doing it is better if you're using a plugin on many pages.
As for your problem, the uri.resource ViewHelper assumes that your resources are in the Resources/Public directory of your extension. So:
<link rel="stylesheet" href="{f:uri.resource(path:'css/bootstrap.css')}">
points to
typo3conf/ext/yourExt/Resources/Public/css/bootstrap.css
It is not possible to point to a file in fileadmin with the resource ViewHelper, see https://git.typo3.org/Packages/TYPO3.CMS.git/blob/HEAD:/typo3/sysext/fluid/Classes/ViewHelpers/Uri/ResourceViewHelper.php.
Current best practise for a TYPO3 page template is to have the template of a TYPO3 Website in an own extension, see e.g. https://github.com/georgringer/modernpackage