Docs: Link optional docs from Apache config

This commit links to the new optional Apache config docs from the Apache
config step in the installation instructions.

Signed-off-by: Remington Steed <rjs7@calvin.edu>

docs/installation/server_installation.txt

@@ -344,6 +344,10 @@ a2ensite eg.conf
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 chown opensrf /var/lock/apache2
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++
+9. Learn more about additional Apache options in the following sections:
+  * <<_apache_rewrite_tricks,Apache Rewrite Tricks>>
+  * <<_apache_access_handler_perl_module,Apache Access Handler Perl Module>>
 
 Configure OpenSRF for the Evergreen application
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