update and split docs

Signed-off-by: CrazyMax <crazy-max@users.noreply.github.com>

docs/advanced/buildkit-config.md

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+# BuildKit daemon configuration
+
+You can provide a [BuildKit configuration](https://github.com/moby/buildkit/blob/master/docs/buildkitd.toml.md)
+to your builder if you're using the [`docker-container` driver](https://docs.docker.com/build/building/drivers/docker-container/)
+(default) with the `config` or `config-inline` inputs:
+
+## Registry mirror
+
+You can configure a registry mirror using an inline block directly in your
+workflow with the `config-inline` input:
+
+```yaml
+name: ci
+
+on:
+  push:
+
+jobs:
+  buildx:
+    runs-on: ubuntu-latest
+    steps:
+      -
+        name: Checkout
+        uses: actions/checkout@v3
+      -
+        name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v2
+        with:
+          config-inline: |
+            [registry."docker.io"]
+              mirrors = ["mirror.gcr.io"]
+```
+
+## Max parallelism
+
+You can limit the parallelism of the BuildKit solver which is particularly
+useful for low-powered machines.
+
+You can use the `config-inline` input like the previous example, or you can use
+a dedicated BuildKit config file from your repo if you want with the
+`config` input:
+
+```toml
+# .github/buildkitd.toml
+[worker.oci]
+  max-parallelism = 4
+```
+
+```yaml
+name: ci
+
+on:
+  push:
+
+jobs:
+  buildx:
+    runs-on: ubuntu-latest
+    steps:
+      -
+        name: Checkout
+        uses: actions/checkout@v3
+      -
+        name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v2
+        with:
+          config: .github/buildkitd.toml
+```

docs/advanced/install-default.md

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+# Install by default
+
+If you want set up the `docker build` command as an alias to
+`docker buildx build`:
+
+```yaml
+name: ci
+
+on:
+  push:
+
+jobs:
+  buildx:
+    runs-on: ubuntu-latest
+    steps:
+      -
+        name: Checkout
+        uses: actions/checkout@v3
+      -
+        name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v2
+        with:
+          install: true
+      -
+        name: Build
+        run: |
+          docker build . # will run buildx
+```

docs/advanced/standalone.md

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+# Standalone mode
+
+If you don't have the Docker CLI installed on the GitHub Runner, Buildx binary
+is invoked directly, instead of calling it as a docker plugin. This can be
+useful if you want to use the `kubernetes` driver in your self-hosted runner:
+
+```yaml
+name: ci
+
+on:
+  push:
+
+jobs:
+  buildx:
+    runs-on: ubuntu-latest
+    steps:
+      -
+        name: Checkout
+        uses: actions/checkout@v3
+      -
+        name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v2
+        with:
+          driver: kubernetes
+      -
+        name: Build
+        run: |
+          buildx build .
+```