Improve docs formatting and fix filename conflicts

[sarahraqueld]

Problems

1. There are a few markdown formatting errors, such as duplicate header and empty links.
2. Cloning this repo on a case-insensitive file system (the default for macOS) results in the following message.

warning: the following paths have collided (e.g. case-sensitive paths
on a case-insensitive filesystem) and only one from the same
colliding group is in the working tree:

  'docs/src/SUMMARY.md'
  'docs/src/summary.md'

Solutions

1. Add links and change header hierarchies
2. Rename summary.md to intro.md

co-authored-by @smaboshe

[sarahraqueld]

The CI is failing with an unrelated issue, so we'll merge this and fix it in a separate PR.