docs: explain how to make scrolling to an anchor smooth (#815)

We also start using this on our documentation site.

solara/server/assets/style.css

@@ -1,6 +1,7 @@
 html {
   /* override vuetify's css reset ress.css */
   overflow-y: auto;
+  scroll-behavior: smooth;
 }
 
 .jupyter-widgets code {

solara/website/pages/documentation/advanced/content/20-understanding/40-routing.md

@@ -227,7 +227,13 @@ The `solara.Link` component also supports linking to HTML elements identified by
 solara.v.Btn(attributes={"id": "my-id"}, ...)
 ```
 
-You can then link to a particular element by appending `#` followed by its id to your link, i.e. `solara.Link(route_or_path="/page#my-id")`.
+You can then link to a particular element by appending `#` followed by its id to your link, i.e. `solara.Link(route_or_path="/page#my-id")`. If you want the page to smoothly scroll to the linked element, you should give an element that contains it (for instance the `html`-element) the following CSS rule:
+
+```css
+html {
+    scroll-behaviour: smooth;
+}
+```
 
 ## Fully manual routing