Standardize on .md extension with MDX processing for all markdown files

Configure Docusaurus to treat all .md files as MDX (format: 'mdx').
This simplifies reviewing since all files use the same .md extension
which renders nicely in GitLab/GitHub previews.

Changes:
1. Set markdown.format: 'mdx' in docusaurus.config.js
2. Renamed all 68 .mdx files to .md (blog + docs + pages)
3. Fixed MDX parsing issues in 6 files by escaping angle brackets:
   - how-many-tuples-can-be-inserted-in-a-page.md: <= to &lt;=
   - how-to-use-openai-apis-in-postgres.md: <100k to &lt;100k
   - from-pgss-to-explain--how-to-find-query-examples.md: <16 to &lt;16
   - how-to-decide-if-query-too-slow.md: <100ms to &lt;100ms
   - uuid-v7-and-partitioning-timescaledb.md: <-> to &lt;-&gt;
   - database-lab-engine-components.md: wrap placeholders in backticks

Build tested locally - passes successfully.

Author: claude