Added ISLR textbook

[sparklycat]

✅️ By submitting this PR, I have verified the following

Guidelines

Content

If adding new advice or content,

• [x] A tool was utilized to check for spelling and grammar.

Resources

If adding a new resource,

• [x] There is a link to the resource.
• [x] There is a description describing what benefits the resource offers.

Formatting

• [x] No whitespace.
• [x] Spaces, not tabs. Specifically, 2 spaces.
• [x] Make sure items in lists are alphabetized.