fixes header nav for installation and plugins; updates README installation URL (#989)

* fixes header nav for installation and plugins

* also updates README for accurate installation URL

---------

Co-authored-by: Chris Lutz <chris@sysadminchris.info>

README.md

@@ -34,7 +34,7 @@ you should also change the secrets.
 `git-secret` [supports `brew`](https://formulae.brew.sh/formula/git-secret), just type: `brew install git-secret`
 
 It also supports `apt` and `yum`. You can also use `make` if you want to.
-See the [installation section](https://git-secret.io/installation) for the details.
+See the [installation section](https://sobolevn.me/git-secret/installation) for the details.
 
 ### Requirements
 

docs/_includes/header.html

@@ -26,9 +26,9 @@
   </div>
 
   <nav class="site-navigation">
-    <a href="/installation">Installation</a>
+    <a href="{{ "/installation" | prepend: site.baseurl }}">Installation</a>
     <a href="{{ "/#commands" | prepend: site.baseurl }}">Commands</a>
-    <a href="/plugins">External plugins</a>
+    <a href="{{ "/plugins" | prepend: site.baseurl }}">External plugins</a>
     <a href="{{ site.github_changelog }}">Changelog</a>
   </nav>