feat(notion-fetch): preserve notion spacers

[luandro]

Summary

• emit for empty Notion paragraphs so intentional spacers survive export
• trim leading/trailing spacer tokens and wire up DocSpacer MDX component
• cover the transformer with unit tests for empty, nested, and populated paragraphs

Testing

• bunx eslint scripts/notionClient.ts scripts/notionClient.test.ts src/components/DocSpacer/index.tsx src/theme/MDXComponents/index.tsx --fix
• bunx eslint scripts/notion-fetch/generateBlocks.ts --fix
• bunx vitest run scripts/notionClient.test.ts

Fixes #49

[luandro]

Summary

• map empty Notion paragraphs to <DocSpacer /> so intentional spacers survive the export
• trim spacer tokens off the top/bottom of generated markdown and add an MDX-wired DocSpacer component
• cover the transformer with unit tests alongside existing Notion client specs

Expected behavior

• Deliberate blank lines in Notion now render as controlled spacing in MDX/docs pages
• No stray spacer components appear at the start or end of generated markdown files

QA instructions

1. Run your usual Notion export pipeline (e.g., bun run notion:fetch) so new content is regenerated.
2. Inspect a page that previously used intentional blank lines—confirm the generated MDX now contains <DocSpacer /> entries instead of dropped spacing.
3. Start the docs locally with bun run dev, navigate to that page, and verify the vertical gaps render without extra spacers at the top or bottom.