Documentation: Spanish translation of perplexity.mdx (#27807)

* Copy perplexity.md file to es/ folder

* Adding perplexity to es/_toctree.yml

* Translate first section

* Calculating PPL section translate

* Example section translate

* fix translate of log-likehood

* Fix title translate

* Fix \ in second paragraph

* Change verosimilitud for log-likelihood

* Run 'make style'

docs/source/es/_toctree.yml

@@ -77,4 +77,6 @@
     title: Filosofía
   - local: bertology
     title: BERTología
+  - local: perplexity
+    title: Perplejidad de los modelos de longitud fija
   title: Guías conceptuales