Reorg design ideas

I've implemented some proposal fixes for various feedback I sent to @gtback and @johnwunder regarding the site redesign, including:
- Removed redundant "Search Data Model" widget from the **Data Model -- Overview** page, since it's already in the site header. (It previously felt 'in the way' between the page heading and the relevant content.)
- Put the **Package** type on top of the rest of the list on that page, in part because it is the only item without the icon, which made it feel out of place in the middle of the list. I feel the justification for this is that the **Package** is the top-level entity in the data model.
- Updated the **Data Model -- Overview** page to alternate the sides that the data types and picture combos are aligned to, and added underlining to headings, to further visually distinguish different sections on the page.
- Added a link to python-stix's readthedocs page to the Documentation page, as users may logically expect it to be there
- Fixed a 404 to the **Documentation -- Concepts -- Data Markings** page
- Right aligned the Data Markings icon on that same page to make it look less out of place (it is the only item with an icon on that page, which looked a bit weird.)
- Make some links, such as external readthedocs links and PDF links open in a new tab
- Web 9.0-ify the "Get In Touch" section on the front page

I'm looking into a couple other fixes to add to this PR, but I'm throwing this up as is for everyone to look at in the interim. Feedback welcome!