Adds RHEL minimum version for ext4 FS support

Also fixes a regression of DOCS-1094

Signed-off-by: Sam Kleinman <samk@10gen.com>
DOCS-1208 agg command overview/xref

Signed-off-by: Sam Kleinman <samk@10gen.com>
minor: refactoring links and fixing a toc tree
theme: fixing jira collector problem