docs: added introduction to docs site

[JoshuaKGoldberg]

PR Checklist

• Addresses an existing open issue: fixes 📝 Documentation: Add introduction paragraph(s) to docs site #145
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

Adds a few explanatory lines and a canonical function example to the docs.

[codecov]

Codecov Report

Merging #149 (e0dda82) into main (e9bc48b) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #149   +/-   ##
=======================================
  Coverage   21.80%   21.80%           
=======================================
  Files          18       18           
  Lines         431      431           
  Branches      171      171           
=======================================
  Hits           94       94           
  Misses        315      315           
  Partials       22       22           

Flag	Coverage Δ
4.2.4	21.11% <ø> (ø)
4.3.5	21.34% <ø> (ø)
4.4.4	21.34% <ø> (ø)
latest	21.34% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[RebeccaStevens]

Looks good to me.
Though maybe we should add a note about if a function appears to be missing, users should check if it exists on typescript itself.