Add stakeholder decision models index page

[sei-ahouseholder]

Is your feature request related to a problem? Please describe.

In the left nav of the HowTo section, there is a heading Stakeholder Decision Models but there is no index page within that block.

Describe the solution you'd like

Add a decision models home page that lists all the existing decision models we have.

Note this is different from #1170 which is about adding reference pages for each decision table.

Acceptance Criteria

• a Stakeholder Decision Models page exists
• page contains links to the various decision models we have
• page is added to mkdocs.yml nav