* add conversion from pdf to latex-like (.mmd) format with nougat

* change vector_db_manager.py to handle .mmd
* add "conversion" tab
* add math mode checkbox in maintab

backend/pdf_to_mmd.py

@@ -0,0 +1,16 @@
+import subprocess
+
+
+def pdf_to_mmd(path_input: str):
+    """
+    Convert a PDF file to MMD format using the Nougat library
+    https://github.com/facebookresearch/nougat
+
+    stream stderr to the front end
+    """
+    output_dir = "../documents/mmds"
+    command = ['nougat', path_input, "-o", output_dir]
+    subprocess.run(command)
+
+
+