feat: add blockquote support to from_markdown() and Post.blockquote()

[ASRagab]

Summary

Adds blockquote (>) support to both the from_markdown() parser and as a convenience method on Post.

Changes

• Post.blockquote(content) — new convenience method following the pattern of heading(), horizontal_rule(), etc. Accepts string or pre-built paragraph content.
• from_markdown() — parses > prefixed lines into blockquote nodes using flush-based grouping (same pattern as bullet lists). Consecutive blockquote lines become a single blockquote with multiple paragraph children. Blank lines between blockquotes create separate blockquote nodes.
• Inline formatting preserved — bold, italic, links all work inside blockquotes via parse_inline().

Tests

Added 10 tests (8 from_markdown scenarios + 2 blockquote() method tests):

• Single-line and multi-line blockquotes
• Separation by blank lines
• Blockquote-to-paragraph transitions
• Inline link preservation inside blockquotes
• Adjacency to bullet lists
• Empty continuation lines
• String input and method chaining

All 16 tests pass (6 existing + 10 new).

Motivation

Blockquotes are one of the most common Markdown elements but were not supported in from_markdown(). This closes that gap while following the existing code patterns.