Add docs for feature definition file format

docs/Docs/documentation/file-formats/feature-definition-file-format.md

@@ -0,0 +1,38 @@
+# Feature Definition File Format
+
+The feature definition file defines the features used in the 
+ranking model. The file format is as follows:
+
+```text
+<line> .=. <featureid> <description>
+<featureid> .=. <positive integer>
+<description> .=. <string>
+```
+
+where
+
+- `<featureid>`
+
+  The feature's unique identifier.
+
+  > [!NOTE]
+  > Feature identifiers should match the identifiers referenced in the training data file.
+
+- `<description>`
+
+  A short description or explanation of the feature.
+
+A feature file can contain comments by starting a line with `#`.
+
+## Example
+
+The following example has five features:
+
+```text
+#   Movies data set features - revision 1
+1   BM25 score for the title field
+2   BM25 score for the untokenized title field
+3   BM25 score for the actors field
+4   BM25 score for the untokenized actors field
+5   Popularity score derived from click data
+```

docs/Docs/documentation/file-formats/toc.yml

@@ -1,6 +1,8 @@
 - name: Training File
   href: training-file-format.md
 - name: Relevance Judgment File
-  href: relevance-judgment-file-format.md  
+  href: relevance-judgment-file-format.md
+- name: Feature Definition File
+  href: feature-definition-file-format.md  
 - name: Ranking File
   href: ranking-file-format.md