Add small text on BibTeX handling (JabRef#11946)

docs/code-howtos/bibtex.md

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+---
+parent: Code Howtos
+---
+# JabRef's handling of BibTeX
+
+The main class to handle a single BibTeX entry is `org.jabref.model.entry.BibEntry`.
+The content of a `.bib` file is handled in `org.jabref.model.database.BibDatabase`.
+Things not written in the `.bib` file, but required for handling are stored in `org.jabref.model.database.BibDatabaseContext`.
+For instance, this stores the mode of the library, which can be BibTeX or `biblatex`.
+
+Standard BibTeX fields known to JabRef are modeled in `org.jabref.model.entry.field.StandardField`.
+A user-defined field not known to JabRef's code is modelled in `org.jabref.model.entry.field.UnknownField`.
+Typically, to get from a String to a `Field`, one needs to use `org.jabref.model.entry.field.FieldFactory#parseField(java.lang.String)`.
+
+## Cross-references
+
+BibTeX allows for referencing other entries by the field `crossref` (`org.jabref.model.entry.field.StandardField#CROSSREF`).
+Note that BibTeX and `biblatex` handle this differently.
+The method `org.jabref.model.entry.BibEntry#getResolvedFieldOrAlias(org.jabref.model.entry.field.Field, org.jabref.model.database.BibDatabase)` handles this difference.