docs: rewrite expected-checksums.md to STE rules

[elharo]

Rewrite expected-checksums.md to ASD-STE100 Simplified Technical English rules. This PR also includes some pretty hefty human rewrites including:

• Sonatype Nexus 2 is EOL and was removed
• Removed paragraph based on incorrect assumption that checksums provided trust and not simply integrity validation

Changes

• Split long sentences into short sentences. No sentence exceeds 25 words.
• Removed banned modal verbs (may, might, should, would, could). Replaced them with can, must, or rewrote the sentence.
• Removed present perfect tenses. Used simple present or simple past instead.
• Removed banned phrases and vague words: utilize, e.g., in order to, basically, naturally, simply, game changer.
• Removed "-ing" verb forms in prose. Kept gerunds that function as nouns or adjectives.
• Split comma-spliced sentences and removed semicolons.
• Standardized vocabulary. One word has one meaning. For example, verify/verification is used consistently for integrity checks.
• Kept one topic per paragraph. No paragraph exceeds six sentences.
• The procedure for the summary checksum file uses the imperative mood with one instruction per step.
• Verified that all code blocks, identifiers, and command line flags are unchanged.