feat: add examples and docs for readers (#323)

examples/data/planets.md

examples/data/stars.docx

examples/readers/README.md

examples/readers/load-assemblyai.ts

examples/readers/load-csv.ts

examples/readers/load-docx.ts

examples/readers/load-html.ts

examples/readers/load-md.ts

examples/readers/load-notion.ts

examples/readers/load-pdf.ts

packages/core/src/index.ts

packages/core/src/tests/readers/DocxReader.test.ts

packages/core/src/tests/readers/MarkdownReader.test.ts