docs: what the CI does (#3305)

.buildkite/README.md

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+# Buildkite
+
+This README provides an overview of the Buildkite pipeline used to automate the build and publishing process.
+
+## Release pipeline
+
+This is the Buildkite pipeline for releasing the APM Agent Java.
+
+### Pipeline Configuration
+
+To view the pipeline and its configuration, click [here](https://buildkite.com/elastic/apm-agent-java-release) or
+go to the definition in the `elastic/ci` repository.
+
+## Snapshot pipeline
+
+This is the Buildkite pipeline for the APM Agent java in charge of the snaposhots.
+
+### Pipeline Configuration
+
+To view the pipeline and its configuration, click [here](https://buildkite.com/elastic/apm-agent-java-snapshot) or
+go to the definition in the `elastic/ci` repository.

.github/workflows/README.md

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+## CI/CD
+
+There are 4 main stages that run on GitHub actions:
+
+* Build
+* Unit Test
+* Integration Test
+* Release
+
+There are some other stages that run for every push on the main branches:
+
+* [Microbenchmark](./microbenchmark.yml)
+* [Snapshoty](./snapshoty.yml)
+* [Sync-branches](./sync-branches.yml)
+
+### Scenarios
+
+* Tests should be triggered on branch, tag and PR basis.
+* Commits that are only affecting the docs files should not trigger any test or similar stages that are not required.
+* Automated release in the CI gets triggered through a GitHub workflow.
+* **This is not the case yet**, but if Github secrets are required then Pull Requests from forked repositories won't run any build accessing those secrets. If needed, then create a feature branch (opened directly on the upstream project).
+
+### How to interact with the CI?
+
+#### On a PR basis
+
+Once a PR has been opened then there are two different ways you can trigger builds in the CI:
+
+1. Git commit based
+1. UI based, any Elasticians can force a build through the GitHub UI
+
+#### Branches
+
+Every time there is a merge to main or any branches the whole workflow will compile and test on Linux and Windows.
+
+### Release process
+
+This process has been fully automated and it gets triggered manually when running the [release](https://github.com/elastic/apm-agent-java/actions/workflows/release.yml) workflow. It runs then a Buildkite pipeline in charge of generating and publishing the artifacts,
+for further details please go to [the buildkite folder](../../.buildkite/README.md).
+
+The tag release follows the naming convention: `v.<major>.<minor>.<patch>`, where `<major>`, `<minor>` and `<patch>`.
+
+### OpenTelemetry
+
+There is a GitHub workflow in charge to populate what the workflow run in terms of jobs and steps. Those details can be seen in [here](https://ela.st/oblt-ci-cd-stats) (**NOTE**: only available for Elasticians).
+
+## Bump automation
+
+[updatecli](https://www.updatecli.io/) is the tool we use to automatically update the specs
+the [APM agents](./updatecli.yml) use.