show Nix logo in the manual (#10442)

the location of files is hard-coded by mdBook.
there is also seems to be no way to define custom templates, therefore
all styling has to be done in the CSS override.

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>

doc/manual/custom.css

@@ -1,3 +1,25 @@
+:root {
+    --sidebar-width: 23em;
+}
+
+h1.menu-title::before {
+  content: "";
+  background-image: url("./favicon.svg");
+  padding: 1.25em;
+  background-position: center center;
+  background-size: 2em;
+  background-repeat: no-repeat;
+}
+
+
+h1.menu-title {
+  padding: 0.5em;
+}
+
+.sidebar .sidebar-scrollbox {
+  padding: 1em;
+}
+
 h1:not(:first-of-type) {
     margin-top: 1.3em;
 }

doc/manual/local.mk

@@ -160,7 +160,7 @@ doc/manual/generated/man1/nix3-manpages: $(d)/src/command-ref/new-cli
 	done
 	@touch $@
 
-$(docdir)/manual/index.html: $(MANUAL_SRCS) $(d)/book.toml $(d)/anchors.jq $(d)/custom.css $(d)/src/SUMMARY.md $(d)/src/command-ref/new-cli $(d)/src/contributing/experimental-feature-descriptions.md $(d)/src/command-ref/conf-file.md $(d)/src/language/builtins.md
+$(docdir)/manual/index.html: $(MANUAL_SRCS) $(d)/book.toml $(d)/anchors.jq $(d)/custom.css $(d)/src/SUMMARY.md $(d)/src/command-ref/new-cli $(d)/src/contributing/experimental-feature-descriptions.md $(d)/src/command-ref/conf-file.md $(d)/src/language/builtins.md $(d)/src/favicon.png $(d)/src/favicon.svg
 	$(trace-gen) \
 		tmp="$$(mktemp -d)"; \
 		cp -r doc/manual "$$tmp"; \

doc/manual/src/favicon.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="587.11" height="516.604" viewBox="0 0 550.416 484.317"><defs><linearGradient id="a"><stop offset="0" style="stop-color:#699ad7;stop-opacity:1"/><stop offset=".243" style="stop-color:#7eb1dd;stop-opacity:1"/><stop offset="1" style="stop-color:#7ebae4;stop-opacity:1"/></linearGradient><linearGradient id="b"><stop offset="0" style="stop-color:#415e9a;stop-opacity:1"/><stop offset=".232" style="stop-color:#4a6baf;stop-opacity:1"/><stop offset="1" style="stop-color:#5277c3;stop-opacity:1"/></linearGradient><linearGradient xlink:href="#a" id="c" x1="200.597" x2="290.087" y1="351.411" y2="506.188" gradientTransform="translate(70.65 -1055.151)" gradientUnits="userSpaceOnUse"/><linearGradient xlink:href="#b" id="e" x1="-584.199" x2="-496.297" y1="782.336" y2="937.714" gradientTransform="translate(864.696 -1491.34)" gradientUnits="userSpaceOnUse"/></defs><g style="display:inline;opacity:1" transform="translate(-132.651 958.04)"><path id="d" d="m309.549-710.388 122.197 211.675-56.157.527-32.624-56.87-32.856 56.566-27.903-.011-14.29-24.69 46.81-80.49-33.23-57.826z" style="opacity:1;fill:url(#c);fill-opacity:1;fill-rule:evenodd;stroke:none;stroke-width:3;stroke-linecap:butt;stroke-linejoin:round;stroke-miterlimit:4;stroke-dasharray:none;stroke-opacity:1"/><use xlink:href="#d" width="100%" height="100%" transform="rotate(60 407.112 -715.787)"/><use xlink:href="#d" width="100%" height="100%" transform="rotate(-60 407.312 -715.7)"/><use xlink:href="#d" width="100%" height="100%" transform="rotate(180 407.419 -715.756)"/><path id="f" d="m309.549-710.388 122.197 211.675-56.157.527-32.624-56.87-32.856 56.566-27.903-.011-14.29-24.69 46.81-80.49-33.23-57.826z" style="color:#000;clip-rule:nonzero;display:inline;overflow:visible;visibility:visible;opacity:1;isolation:auto;mix-blend-mode:normal;color-interpolation:sRGB;color-interpolation-filters:linearRGB;solid-color:#000;solid-opacity:1;fill:url(#e);fill-opacity:1;fill-rule:evenodd;stroke:none;stroke-width:3;stroke-linecap:butt;stroke-linejoin:round;stroke-miterlimit:4;stroke-dasharray:none;stroke-dashoffset:0;stroke-opacity:1;color-rendering:auto;image-rendering:auto;shape-rendering:auto;text-rendering:auto;enable-background:accumulate"/><use xlink:href="#f" width="100%" height="100%" style="display:inline" transform="rotate(120 407.34 -716.084)"/><use xlink:href="#f" width="100%" height="100%" style="display:inline" transform="rotate(-120 407.288 -715.87)"/></g></svg>