Add more files to get compat with Grav.

This structure may look a little verbose, but it's very powerful.

fr/01.optique/03.la-nature-de-la-lumiere/02.antiseche/cheatsheet.md

+## Trucs & Astuces
+
+Lorem ipsum…
\ No newline at end of file

fr/01.optique/03.la-nature-de-la-lumiere/columns.md

+---
+content:
+    # https://learn.getgrav.org/15/content/collections#summary-of-collection-options
+    items: @self.children
+---
\ No newline at end of file

fr/01.optique/topic.md

+---
+content:
+    # https://learn.getgrav.org/15/content/collections#summary-of-collection-options
+    items: @self.children
+---
\ No newline at end of file

fr/topics.md

+---
+content:
+    # https://learn.getgrav.org/15/content/collections#summary-of-collection-options
+    items: @self.children
+---
\ No newline at end of file