chore: Update documentation heading border styles

What does this MR do?

Update heading border-bottom styles to be consistent with tab border color.

Tighten padding-bottom to improve vertical rhythm between headings and related content.

Changes

• Update $border-color to $gray-100 replacing $border-color--light
• Remove border-bottom and padding-bottom from <h1> typography
• Update padding-bottom spacing on <h2> typography
• Update <blockquote> styles remove background-color and padding
• Add v-if condition to <p> rendering for page.description
• Update template page header classes for consistent rendering and spacing

Closes #1462 (closed)

Screenshots

Before	After