Add a "fees" section to the ERC4626 guide (#4054)

Co-authored-by: Francisco Giordano <fg@frang.io>

scripts/prepare-docs.sh

@@ -12,4 +12,12 @@ rm -rf "$OUTDIR"
 
 hardhat docgen
 
+# copy examples and adjust imports
+examples_dir="docs/modules/api/examples"
+mkdir -p "$examples_dir"
+for f in contracts/mocks/docs/*.sol; do
+  name="$(basename "$f")"
+  sed -e '/^import/s|\.\./\.\./|@openzeppelin/contracts/|' "$f" > "docs/modules/api/examples/$name"
+done
+
 node scripts/gen-nav.js "$OUTDIR" > "$OUTDIR/../nav.adoc"