atom/rss quickfix, CI to auto-test rss/atom correctness, and misc improvements

Blog post refinements and RSS feed validation

Changes

Blog post improvements:

• Better URL slug: Added descriptive slug with redirect from old short URL for improved SEO
• Updated timestamp: Changed post time from 13:59:59 to 23:59:59
• Clarified pgBackRest purpose: Added explanation that pgBackRest is primarily a backup tool being repurposed for replica creation
• Removed Credit paragraph: Cleaned up redundant credit section (Maxim remains as co-author in frontmatter)

RSS/Atom feed fixes:

• Added automated feed validation to CI: New validate_feeds job runs before build/deploy
• Validates XML syntax for rss.xml and atom.xml using xmllint
• Validates JSON syntax for feed.json using jq
• Saves failed feeds as artifacts for debugging
• Fixed XML parsing errors: Changed tag "Guides & best practices" to "Guides and best practices"
• Unescaped ampersands in tag names were causing xmlParseEntityRef errors in Atom feed

Benefits

• Better SEO with descriptive URL containing keywords
• Prevents broken feeds through CI validation before production
• No breaking changes: old URL redirects to new one automatically
• Cleaner content with removed redundant information

Files changed

• blog/20251105-postgres-marathon-2-012.mdx
• .ci/validate-feeds.sh (new)
• .gitlab-ci.yml
• docusaurus.config.js
• src/theme/BlogSidebar/Desktop/index.tsx
• src/theme/BlogSidebar/Mobile/index.tsx
• 6 blog post files with updated tag names