Docs: replace deprecated .lead with .fs-lg .fw-light in homepage and examples

[julien-deramond]

Description

Spotted in #42487.

This PR replaces the deprecated .lead text utility with .fs-lg .fw-light in homepage and examples.

In v5:

.lead {
  font-size: 1.25rem; // 20px
  font-weight: 300;
}

The closest rendering we can get with v6 utilities is .fs-lg (24px) and .fw-light (300); a bit bigger.

What's been done:

• All examples have been updated to make this replacement
  • Special mention in Checkout example that sill showed the .lead rendering/explanation
• Updated the homepage to use this replacement, and also removed the .lead overrides. I simplified everything because I know that @mdo will revamp this homepage anyway.
• Added a note in the migration guide to mention this change

[coliff]

looking good - let's get it merged!