Add Jenkinsfiles

[tyler-yankee]

Towards #22826. Adds .jenkins/ for defining Jenkins job logic in Drake itself, rather than dynamically generating it in job definitions:

• Adds production, experimental, staging, cache server health check, and external examples pipelines in Jenkinsfiles
• Adds shared utils in utils/utils.groovy
• Fixes documentation in jenkins.md for how to use the new experimental jobs

The continuous, nightly, experimental, and pre-merge jobs will all use these files. Experimental and pre-merge jobs will look a little different, but function the same way in terms of providing parameters to run branches/PRs of drake and CI, running automatically on PRs, keeping the "@drake-jenkins-bot ..." comment triggers, etc.

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This change is 

[tyler-yankee]

.jenkins/Jenkinsfile line 72 at r1 (raw file):

// Performs the main build step by calling into the drake-ci driver script
// with the necessary credentials and environment variables.
def doBuild(Map scmVars) {

Working: This function needs an additional boolean parameter for staging which defines DRAKE_VERSION as an environment variable, per the existing staging jobs.

[tyler-yankee]

-(status: do not review) +@BetsyMcPhail for feature review, please.

[tyler-yankee]

.jenkins/Jenkinsfile-experimental line 61 at r3 (raw file):

// Loads utils by performing a sparse checkout to WORKSPACE/jenkins-utils.
def fetchUtils() {
  def thisBranch = scm.branches[0].name

Working: this is broken for multibranch projects, because it doesn't refer to it with the nice SHA syntax (pr/xxx/head) but a more "display"-ish name (PR-xxx).

[tyler-yankee]

@drake-jenkins-bot linux-jammy-gcc-bazel-experimental-release-multibranchtest2 please

[tyler-yankee]

@drake-jenkins-bot linux-jammy-gcc-bazel-experimental-release-test please
@drake-jenkins-bot mac-arm-sonoma-clang-bazel-experimental-release-test please
@drake-jenkins-bot linux-jammy-experimental-cache-server-health-check-test please

[tyler-yankee]

@drake-jenkins-bot linux-jammy-gcc-bazel-experimental-release please
@drake-jenkins-bot mac-arm-sonoma-clang-bazel-experimental-release please
@drake-jenkins-bot linux-jammy-experimental-cache-server-health-check please

Unfortunately I didn't write in the regex matching with "test", so this comment is going to trigger both the regular experimental jobs and the new test ones. 🤷

Edit: Interestingly, the first one caught the regular job, while the last two caught the test jobs. Possibly a race condition with the plugin? This isn't a regular use-case so I'm not too concerned about it. Good to know that the mac one caught on the first build, so it doesn't have the oddity that the continuous jobs do where you have to run them once manually before the triggers work.