docs: add links to tutorials on landing page for documentation

[benwandrew]

Description

adds links to corresponding feature and usage pages on the docs landing page.

• resolves #460

Type of change

• docs: Documentation only changes

Remarks (Optional)

• will need to add more links as more individual docs pages are created, so keeping open as a draft for now.

[benwandrew]

@younesStrittmatter adding a link for "Workflow logic" under Features is a bit different in this case since we don't include a workflow/index.md in the docs directory of this repo. can we somehow link to the top-level page that will be built via the !import in the mkdocs.yml (ie, https://autoresearch.github.io/autora/user-guide/workflow/)? or should we alter the workflow section of the nav to have a Home: workflow/index.md and then import the individual sections (Interactive, Command Line)? sorry if that doesn't make sense... maybe i can ask live at some point?