Add instructions to embed code samples in docs

[geriom]

Changes

Add instructions to embed code samples from external files in the documentation.

Submitter Checklist

These are the criteria that every PR should meet, please check them off as you review them:

• [x] Includes docs (if user facing)
• [x] Commit messages follow commit message best practices

See the contribution guide for more details.

[geriom]

The red highlighting is a bit distracting but I don't think it's a blocker

The code highlighter is doing that, it does look ugly. Switched language to html instead, looks better now.

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: AlanGreene, chitrangpatel

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/tektoncd/website/blob/main/OWNERS)~~ [AlanGreene] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment