feat: new pdf parsers: docling and pymupdf4llm

See full list of available PdfParsingConfig options here:

https://github.com/langroid/langroid/blob/main/langroid/parsing/parser.py

Also:

• removing pdfplumber due to outdated + conflicting dependency constraints.
• update to newwer OpenAI embeddings models announced 25 Jan 2025 (text-embedding-3-small, ...-large)