Fix two broken site links and the wrong mailing list

The 'Books > Writing Components' menu entry pointed at
/writing-components-trail/index.html, which does not exist. Because it
lives in site.xml it was broken on every page of the site. Its only
child duplicated the working 'Writing Plexus Components' entry directly
below it, so the Books group is removed rather than repointed.

index.apt linked to ./faq.html, which also does not exist. There is no
FAQ to point at, so the sentence goes.

Community > Mailing Lists pointed at groups.google.com/g/mojohaus-dev,
which belongs to MojoHaus. Plexus has no mailing list; issues are
handled per repository on GitHub, so the entry now says so.

Verified by building the site and walking every internal link: both
targets are gone and nothing new broke.

Part of codehaus-plexus/.github#58

Author: slachiewicz

src/site/apt/index.apt

@@ -74,6 +74,4 @@ Plexus Container and Companion Tools History
   The Plexus project provides a number of pre-built components for common tasks and toolkits
   such as Velocity, i18n, and {{{./ref/available-components.html} many more}}.
 
-  For answers to common questions about Plexus, see the {{{./faq.html} FAQ}}.
-
 

src/site/site.xml

@@ -47,10 +47,6 @@ under the License.
     </menu>
 
     <menu name="Old IoC Documentation">
-    <item name="Books">
-      <item name="Writing Components" href="/writing-components-trail/index.html" />
-    </item>
-
     <item name="Guides">
       <!-- 5 minute tutorial -->
       <item name="Quick Start" href="/guides/quick-start/getting-started.html" />
@@ -74,7 +70,7 @@ under the License.
     </menu>
 
     <menu name="Community">
-      <item name="Mailing Lists" href="https://groups.google.com/g/mojohaus-dev" />
+      <item name="Issues" href="https://github.com/codehaus-plexus" />
       <item name="Get Involved" href="/get-involved.html" />
     </menu>
   </body>