Add prefer-blockquote-vs-docsy-alerts rule, apply rule to en pages, plus some copyedits

[chalin]

• Contributes to Provide agent-friendly content & support #9449
• Adds a custom markdownlint rule that flags Docsy alert shortcode calls
• Converts some alert shortcode calls that have creeped in.
  • For the most part, I've not edited the alert title or text, except the one in Go Sample (see inline comment)
• Registers the rule in .markdownlint-cli2.yaml and keeps it disabled in the root .markdownlint.yaml with a short comment
• Enables the rule only under content/en through content/en/.markdownlint.yaml
• Documents intended behavior, example patterns, and Hugo escaped shortcode delimiters in scripts/_md-rules/prefer-blockquote-vs-docsy-alerts/README.md with a link to Hugo’s escaping documentation
• Adds Node tests for normal calls, fenced and indented cases, inline literals, and commented Hugo delimiter forms
• Suppresses the rule on the Spring Boot annotations page where inline shortcode examples are intentional
• Adds test:local-tools npm script, documents it in site docs.
  • Note: that this will be run as a part of CI via test:compound-tests
• Preview: e.g., https://deploy-preview-9659--opentelemetry.netlify.app/docs/languages/go/sampling/#custom-samplers

[chalin]

@tiffany76 @open-telemetry/go-approvers et all: note the copyedits applied for this alert. Let me know if you are ok with them.

[chalin]

For context, the original alert prose was added via:

• Add guidance for custom samplers #8756

/cc @pratik-mahalle @vitorvasc @svrnm

[chalin]

Thanks @vitorvasc.

@tiffany76 @open-telemetry/go-approvers et all: note the copyedits applied for this alert. Let me know if you are ok with them.

Will merge now. Feedback is still welcome.