fix: Fact-check of misc standalone pages

[Swiftaxe]

Summary

Fact-checked, removed noise, and added frontmatter to 5 standalone pages.

All 5 files:

• Added missing description frontmatter with primary keyword leading for SEO

12-file-uploads:

• "RAM memory" → "RAM"
• "It's possible to quickly check if an uploaded file exists or access the file itself..." → single concise sentence

13-health-checks:

• "Sometimes it is helpful to add custom health metrics. This can be for monitoring external services..." → "Add custom health metrics to monitor..."

20-shared-packages:

• "follow the steps below" → colon
• Removed exclamation mark from closing sentence

21-security-configuration:

• Removed all unnecessary bold from plain text in the intro section
• Removed near-duplicate intro sentence ("To enable SSL/TLS in Serverpod, configure the SecurityContextConfig..." was already stated on the line above)

22-experimental:

• Replaced 3-sentence marketing preamble with 1 direct sentence
• Fixed "one ore more" → "one or more" typo