Add documentation about inheritance for template.html

Author: kkaris

indra/assemblers/html/templates/indra/inheritence.md

@@ -0,0 +1,20 @@
+The file template.html in this directory is template base for several files. See
+the diagram below for the inheritance structure. If you make changes to
+template.html, be sure to check all the files that inherit from it to make sure
+they still work as expected.
+
+These files are currently in active use, however there are more files that
+inherit from template.html that are not currently in use.
+- search.html is the template used for https://db.indra.bio
+- search_statements.html is the template used for https://db.indra.bio/statements
+- statements_view.html is the template used in the HtmlAssembler class in indra.
+- idbr_statements_view.html is the template used for https://db.indra.bio/statements/from...
+
+```mermaid
+graph LR
+    A[indra: template.html] --> B[indra_db: idbr_template.html]
+    A --> C[indra: statements_view.html - serves HtmlAssembler]
+    C --> D[indra_db: idbr_statements_view.html - serves db.indra.bio/statements/from...]
+    B --> E[indra_db: search.html - serves db.indra.bio]
+    B --> F[indra_db: search_statements.html - serves db.indra.bio/statements search]
+```