Introduce .secondary-nav and handle .page-content spacing universally (#29982)

Fixes: https://github.com/go-gitea/gitea/issues/29981. Introduce
`.secondary-nav` as a universal way for styling and margin adjustments
inside `.page-content`.

If the first child of `.page-content` is `.secondary-nav`, we add margin
below it, otherwise we add padding to the first child. Notable changes:

- `--color-header-wrapper` is replaced with `--color-secondary-nav-bg`.
- `navbar` class is removed.

---------

Co-authored-by: Giteabot <teabot@gitea.io>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

templates/explore/navbar.tmpl

@@ -1,4 +1,4 @@
-<overflow-menu class="ui secondary pointing tabular top attached borderless menu navbar">
+<overflow-menu class="ui secondary pointing tabular top attached borderless menu secondary-nav">
 	<div class="overflow-menu-items tw-justify-center">
 		<a class="{{if .PageIsExploreRepositories}}active {{end}}item" href="{{AppSubUrl}}/explore/repos">
 			{{svg "octicon-repo"}} {{ctx.Locale.Tr "explore.repos"}}