Replace Beats/Agent 'has not yet been released' with install commands

[kilfoyle]

During migration the install steps for Elastic Agent and Beats were replaced with the "this version has not yet been released" string that was used conditionally for yet-to-be released versions. So, this adds back in the actual install steps.

See related PR for Beats docs:

original

fixed

Examples

• Tutorial Monitor a Java Application:
  - Install and configure Filebeat tabs
  - Install and configure Metricbeat tabs
  - Setup tabs with commands for Heartbeat
• Install standalone Elastic Agent:
  • The "download and extract the installation package" tabs in Step 1.

Closes: #853

[eedugon]

Similar fixes than in the other PR, with a few comments.

I believe we should create snippets to not duplicate the same so many times, but maybe at a later stage.

[eedugon]

@kilfoyle , I'm getting a 404 when trying to open the Elastic Agent preview link: https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/854/reference/ingestion-tools/fleet/install-standalone-elastic-agent

Just FYI, not sure why!

[kilfoyle]

I'm getting a 404 when trying to open the Elastic Agent preview link

Oops. I think that was just some clumsy copying-and-pasting on my part. I've fixed the link in the PR description.

[eedugon]

LGTM, the only issue I see is that the {{stack-version}} is not being substituted in the Windows tabs, just FYI:

[kilfoyle]

the only issue I see is that the {{stack-version}} is not being substituted in the Windows tabs, just FYI

Nice catch @eedugon! It seems that substitutions aren't supported inside of single backtick code strings, but that's okay. Since the instruction isn't a command, but rather just to rename a directory, I've changed {{stack-versions}} with [version] in f92c1ea.

Thanks for the approval!