Streamline onboarding process

Currently the [issue template](https://github.com/bazelbuild/continuous-integration/blob/master/.github/ISSUE_TEMPLATE/adding-your-project-to-bazel-ci.md) only asks for the existence of a presubmit configuration.

However, there are actually more steps for onboarding a project that depend on the project maintainer:
1. We need their email in order to add them to the Buildkite org.
2. They need to set up webhooks, but the instructions are only available once the pipeline exists and they have access to it.
3. Buildkite can only update GitHub commit statuses once the project maintainer either connects their GitHub account to Buildkite, or once they give github.com/bazel-io write permissions to their repo. In the latter case there's also a difference between personal repositories and org-owned repos.

Accepting the invitation is also problematic since bazel-io's emails are not easily accessible.

As a first step we should improve the issue template and/or the documentation.

Start with .github/ISSUE_TEMPLATE/adding-your-project-to-bazel-ci.md, then inspect the related onboarding documentation and the current Buildkite access instructions. Update the template and/or documentation to cover maintainer email, webhook setup, GitHub status permissions, and accepting invitations; done means each maintainer-dependent step and its relevant repository case is explained.

Written by the indexing model from the issue text.