Populate `.env.example` with examples and instructions (#52912)

.env.example

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+# This file is a template for what your untracked .env file might look like for local development.
+# Please copy this to a new .env file and fill in the values as needed.
+
+# Requires a running local Elasticsearch service. Can be started via Docker, see https://github.com/github/docs-engineering/blob/main/docs/elasticsearch/elasticsearch-locally.md
+# When this value is unset searches will be proxied to the production Elasticsearch endpoint
+ELASTICSEARCH_URL=http://localhost:9200
+
+# Set for sending events in local development. See https://github.com/github/docs-engineering/blob/main/docs/analytics/hydro-mock.md
+HYDRO_ENDPOINT=
+HYDRO_SECRET=
+
+# Localization variables
+# See https://github.com/github/docs-internal/tree/main/src/languages#working-with-translated-content-locally
+ENABLED_LANGUAGES=
+TRANSLATIONS_ROOT=
+
+# For running the src/search/scripts/scrape script
+# You may want a lower value depending on your CPU
+BUILD_RECORDS_MAX_CONCURRENT=100
+BUILD_RECORDS_MIN_TIME=
+
+# Set to true to enable the /fastly-cache-test route for debugging Fastly headers
+ENABLE_FASTLY_TESTING=