Capitalize section headers in docs

[froschdesign]

Updated documentation to consistently capitalize section headers across multiple validator Markdown files. This change ensures uniformity and improves readability throughout the docs.

Reference:

Headlines

Headlines should be used to break longer text sections to brighten up.

For headlines the Chicago Manual of Style capitalization rules must be used:

1. Capitalize the first and the last word.
2. Capitalize nouns, pronouns, adjectives, verbs, adverbs, and subordinate conjunctions.
3. Lowercase articles (a, an, the), coordinating conjunctions, and prepositions.
4. Lowercase the ‘to’ in an infinitive (I want to play guitar).

Helper

The online tool capitalizemytitle.com can help to capitalize and case convert the headlines.

[gsteel]

LGTM 👍