docs: add 'Active links' (#2183)

* docs: add RouterLink classes

* Rename page to 'Active links'

packages/docs/.vitepress/config/en.ts

@@ -108,6 +108,10 @@ export const enConfig: LocaleSpecificConfig<DefaultTheme.Config> = {
               text: 'Passing Props to Route Components',
               link: '/guide/essentials/passing-props.html',
             },
+            {
+              text: 'Active links',
+              link: '/guide/essentials/active-links.html',
+            },
             {
               text: 'Different History modes',
               link: '/guide/essentials/history-mode.html',

packages/docs/guide/essentials/active-links.md

@@ -0,0 +1,78 @@
+# Active links
+
+It's common for applications to have a navigation component that renders a list of RouterLink components. Within that list, we might want to style links to the currently active route differently from the others.
+
+The RouterLink component adds two CSS classes to active links, `router-link-active` and `router-link-exact-active`. To understand the difference between them, we first need to consider how Vue Router decides that a link is _active_.
+
+## When are links active?
+
+A RouterLink is considered to be ***active*** if:
+
+1. It matches the same route record (i.e. configured route) as the current location.
+2. It has the same values for the `params` as the current location.
+
+If you're using [nested routes](./nested-routes), any links to ancestor routes will also be considered active if the relevant `params` match.
+
+Other route properties, such as the [`query`](../../api/interfaces/RouteLocationNormalized#query), are not taken into account.
+
+The path doesn't necessarily need to be a perfect match. For example, using an [`alias`](./redirect-and-alias#Alias) would still be considered a match, so long as it resolves to the same route record and `params`.
+
+If a route has a [`redirect`](./redirect-and-alias#Redirect), it won't be followed when checking whether a link is active.
+
+## Exact active links
+
+An ***exact*** match does not include ancestor routes.
+
+Let's imagine we have the following routes:
+
+```js
+const routes = [
+  {
+    path: '/user/:username',
+    component: User,
+    children: [
+      {
+        path: 'role/:roleId',
+        component: Role,
+      }
+    ]
+  }
+]
+```
+
+Then consider these two links:
+
+```vue-html
+<RouterLink to="/user/erina">
+  User
+</RouterLink>
+<RouterLink to="/user/erina/role/admin">
+  Role
+</RouterLink>
+```
+
+If the current location path is `/user/erina/role/admin` then these would both be considered _active_, so the class `router-link-active` would be applied to both links. But only the second link would be considered _exact_, so only that second link would have the class `router-link-exact-active`. 
+
+## Configuring the classes
+
+The RouterLink component has two props, `activeClass` and `exactActiveClass`, that can be used to change the names of the classes that are applied:
+
+```vue-html
+<RouterLink
+  activeClass="border-indigo-500"
+  exactActiveClass="border-indigo-700"
+  ...
+>
+```
+
+The default class names can also be changed globally by passing the `linkActiveClass` and `linkExactActiveClass` options to `createRouter()`:
+
+```js
+const router = createRouter({
+  linkActiveClass: 'border-indigo-500',
+  linkExactActiveClass: 'border-indigo-700',
+  // ...
+})
+```
+
+See [Extending RouterLink](../advanced/extending-router-link) for more advanced customization techniques.