docs: put back docstring for fnl template

src/ccbr_tools/templates/_extensions/fnl/__init__.py

@@ -0,0 +1,20 @@
+"""
+Quarto HTML format with FNL branding guidelines
+
+First run `ccbr_tools quarto-add fnl`, then modify the `_quarto.yml` file with the following:
+
+```yaml
+website:
+  ...
+  page-footer:
+    background: black
+    foreground: white
+    left: |
+      [![](/_extensions/fnl/fnl-logo-dark.png){height=70px}](https://frederick.cancer.gov/research/science-areas/bioinformatics-and-computational-science/advanced-biomedical-computational-science)
+    center: |
+      Created by the
+      [CCR Collaborative Bioinformatics Resource](https://github.com/CCBR)
+
+format: fnl-html
+```
+"""