Add model cards for DeepPavlov models (#3138)

* add empty model cards for every current DeepPavlov model

* fix: replace cyrillic `с` with `c`

* docs: add model cards for current DeepPavlov BERT models

* docs: add links for arXiv preprints

model_cards/DeepPavlov/bert-base-bg-cs-pl-ru-cased/README.md

@@ -0,0 +1,18 @@
+---
+language:
+- bulgarian
+- czech
+- polish
+- russian
+---
+
+# bert-base-bg-cs-pl-ru-cased
+
+SlavicBERT\[1\] \(Slavic \(bg, cs, pl, ru\), cased, 12-layer, 768-hidden, 12-heads, 180M parameters\) was trained
+on Russian News and four Wikipedias: Bulgarian, Czech, Polish, and Russian.
+Subtoken vocabulary was built using this data. Multilingual BERT was used as an initialization for SlavicBERT.
+
+
+\[1\]: Arkhipov M., Trofimova M., Kuratov Y., Sorokin A. \(2019\).
+[Tuning Multilingual Transformers for Language-Specific Named Entity Recognition](https://www.aclweb.org/anthology/W19-3712/).
+ACL anthology W19-3712.