Update website

[pzivich]

Reorganization:

• [x] Create Causal page
• [x] Move IPW to Causal page
• [x] Add time-fixed g-formula doc
• [x] Add time-varying g-formula doc
• [x] Add doubly robust doc
• [x] Add IPMW doc
• [x] Move dynamic risk plot to graphics (better suited there since it is under that branch)
• [x] Update index.rst
• [x] Change LOESS value for functional form plots (I set it too small so it looks silly)
• [x] Condense p-values plots
• [x] Consider switching forest plot from my study to causal methods instead
• [x] Remove histogram from diagnostics
• [x] Update README.md to reference all causal method implemented