Improve layout of the docs

[drbenvincent]

• Change Nonequivalent group designs heading to ANCOVA
• Add new Interrupted Time Series section
• Move excess deaths and Example 2 notebooks into the new ITS section
• Check name of those notebooks, eg sc -> its and change wording in notebooks (if necessary) to focus on ITS rather than SC.
• Banking dataset notebook
  • Change subheading of Analysis 1 to something like Analysis 1 - 2x2 DiD,
  • Change Analysis 2 to something like Analysis 2 - DiD with multiple pre/post observations.
  • Under the Analysis 2 section, could also add some brief text to say that DiD with multiple pre/post observations is getting quite close to cITS. But we don't have a proper example for that yet.

• Make sure to build the docs locally to check it's all as expected.
• Submit a PR which refers to this issue number and request Ben to review :)