Add markdownlint CI for blog posts

[TineoC]

Run markdownlint on new/modified blog posts in any language during pull requests.

Refs: kubernetes/website#54795

[kubernetes-prow]

[APPROVALNOTIFIER] This PR is APPROVED

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[TineoC]

/hold

[TineoC]

=== PR #796 — Draft: Add blog editor onboarding guide ===
stdin:73 error MD040/fenced-code-language Fenced code blocks should have a language specified [Context: "```"]
stdin:116:17 error MD009/no-trailing-spaces Trailing spaces [Expected: 0 or 2; Actual: 1]
stdin:131 error MD036/no-emphasis-as-heading Emphasis used instead of a heading [Context: "ALWAYS CHECK ON MOBILE TOO"]
stdin:168:39 error MD009/no-trailing-spaces Trailing spaces [Expected: 0 or 2; Actual: 1]
stdin:191:19 error MD026/no-trailing-punctuation Trailing punctuation in heading [Punctuation: '!']
stdin:221 error MD012/no-multiple-blanks Multiple consecutive blank lines [Expected: 1; Actual: 2]
stdin:223:21 error MD047/single-trailing-newline Files should end with a single newline character

=== PR #795 — Draft: KCD NY ===
stdin:17 error MD012/no-multiple-blanks Multiple consecutive blank lines [Expected: 1; Actual: 2]
stdin:18:13 error MD009/no-trailing-spaces Trailing spaces [Expected: 0 or 2; Actual: 1]
stdin:18 error MD022/blanks-around-headings Headings should be surrounded by blank lines [Expected: 1; Actual: 0; Below] [Context: "## Picture-1"]
stdin:21:13 error MD009/no-trailing-spaces Trailing spaces [Expected: 0 or 2; Actual: 1]
stdin:21 error MD022/blanks-around-headings Headings should be surrounded by blank lines [Expected: 1; Actual: 0; Below] [Context: "## Picture-2"]
stdin:24 error MD022/blanks-around-headings Headings should be surrounded by blank lines [Expected: 1; Actual: 0; Below] [Context: "## Picture-3"]

[TineoC]

/unhold

[TineoC]

@jberkus @lmktfy

if this is too invasive, do you think setting up these rules as warnings on the CI Checks would be better?

To not block blogs from being published as we try this tool

[kubernetes-prow]

PR needs rebase.

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