Add opentelemetry.io docs (open-telemetry#2839)

Per open-telemetry/opentelemetry.io#472, we're mirroring the docs content on the website to each SIG. When a release occurs and these docs are updated, please make an issue or PR mirroring them to their appropriate location in the website repo (https://github.com/open-telemetry/opentelemetry.io/tree/main/content/en/docs/collector).

.github/workflows/docs-update.yml

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+name: Update OpenTelemetry Website Docs
+
+on:
+  # triggers only on a manual dispatch
+  workflow_dispatch:
+
+jobs:
+  update-docs:
+    runs-on: ubuntu-latest
+    steps:
+      - name: checkout
+        uses: actions/checkout@v2.3.4
+      - name: make-pr
+        env:
+          API_TOKEN_GITHUB: ${{secrets.DOC_UPDATE_TOKEN}}
+          # Destination repo should always be 'open-telemetry/opentelemetry.io'
+          DESTINATION_REPO: open-telemetry/opentelemetry.io
+          # Destination path should be the absolute path to your language's friendly name in the docs tree (i.e, 'content/en/docs/java')
+          DESTINATION_PATH: content/en/docs/collector
+          # Source path should be 'website_docs', all files and folders are copied from here to dest
+          SOURCE_PATH: website_docs
+        run: |
+          TARGET_DIR=$(mktemp -d)
+          export GITHUB_TOKEN=$API_TOKEN_GITHUB
+          git config --global user.name austinlparker
+          git config --global user.email austin@lightstep.com
+          git clone "https://$API_TOKEN_GITHUB@github.com/$DESTINATION_REPO.git" "$TARGET_DIR"
+          rsync -av --delete "$SOURCE_PATH/" "$TARGET_DIR/$DESTINATION_PATH/"
+          cd "$TARGET_DIR"
+          git checkout -b docs-$GITHUB_REPOSITORY-$GITHUB_SHA
+          git add .
+          git commit -m "Docs update from $GITHUB_REPOSITORY"
+          git push -u origin HEAD:docs-$GITHUB_REPOSITORY-$GITHUB_SHA
+          gh pr create -t "Docs Update from $GITHUB_REPOSITORY" -b "This is an automated pull request." -B main -H docs-$GITHUB_REPOSITORY-$GITHUB_SHA
+          echo "done"
+          

website_docs/_index.md

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+---
+title: "Collector"
+linkTitle: "Collector"
+weight: 10
+description: >
+  <img width="35" src="https://raw.github.com/open-telemetry/opentelemetry.io/main/iconography/32x32/Collector.svg"></img>
+  Vendor-agnostic way to receive, process and export telemetry data
+---
+
+<img src="https://raw.github.com/open-telemetry/opentelemetry.io/main/iconography/Otel_Collector.svg"></img>
+
+The OpenTelemetry Collector offers a vendor-agnostic implementation on how to
+receive, process and export telemetry data. It removes the need to run,
+operate, and maintain multiple agents/collectors in order to support
+open-source observability data formats (e.g. Jaeger, Prometheus, Fluent Bit,
+etc.) sending to one or more open-source or commercial back-ends. The Collector
+is the default location instrumentation libraries export their telemetry data.
+
+Objectives:
+
+- Usable: Reasonable default configuration, supports popular protocols, runs and collects out of the box.
+- Performant: Highly stable and performant under varying loads and configurations.
+- Observable: An exemplar of an observable service.
+- Extensible: Customizable without touching the core code.
+- Unified: Single codebase, deployable as an agent or collector with support for traces, metrics, and logs (future).