chore: add downloads in README, security policy and update ci actions (#401)

* add security policy

Signed-off-by: Michele Dolfi <dol@zurich.ibm.com>

* update deprecated actions

Signed-off-by: Michele Dolfi <dol@zurich.ibm.com>

* add comment about licenses for new dependencies

Signed-off-by: Michele Dolfi <dol@zurich.ibm.com>

* add pypi downloads badge

Signed-off-by: Michele Dolfi <dol@zurich.ibm.com>

* add citation file

Signed-off-by: Michele Dolfi <dol@zurich.ibm.com>

---------

Signed-off-by: Michele Dolfi <dol@zurich.ibm.com>

CITATION.cff

@@ -0,0 +1,15 @@
+# This CITATION.cff file was generated with cffinit.
+# Visit https://bit.ly/cffinit to generate yours today!
+
+cff-version: 1.2.0
+title: Docling
+message: 'If you use Docling, please consider citing as below.'
+type: software
+authors:
+  - name: Docling Team
+identifiers:
+  - type: url
+    value: 'https://arxiv.org/abs/2408.09869'
+    description: 'arXiv:2408.09869'
+repository-code: 'https://github.com/DS4SD/docling'
+license: MIT