Fix file name collision on case-insensitive file systems

[mattbrictson]

Problem

Cloning the thoughtbot/factory_bot 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'

This makes it difficult to contribute to this project from a typical Mac setup.

Solution

SUMMARY.md is a special file for mdbook, so it must stay as is.

To solve the collision, this PR renames summary.md to intro.md. This has two benefits:

1. intro.md no longer collides with SUMMARY.md, meaning the repo can be cloned without warnings.
2. intro.md better aligns with the contents of that file, which has the name "Intro" in the table of contents.

I updated the table of contents in SUMMARY.md to reflect the new file name, and added a redirect for summary.html → intro.html for users and search engines that might be holding on to the old URL.

Fixes #1580