Speed up or avoid downloading of css from http://www.php.net in render.php

[TysonAndre]

When running phd/render.php --docbook doc-base/.manual.xml --package PHP --partial=book.simdjson --format xhtml - it takes 10 seconds

Using https://github.com/adsr/phpspy to render a flame graph:

1. Use php's curl module if available?
2. Use a different CDN for static assets if possible, or check if the file is recent? (This is taking 5 seconds per file, which is a different question for a different repo)
3. Send etags/if-modified-since? (doesn't help, still slow https://daniel.haxx.se/blog/2019/12/06/curl-speaks-etag/ )
4. It's a fast 0.01 second redirect to https://www.php.net/styles/theme-base.css
5. Maybe check the modification time on disk and cache on disk as sha($url) on http 200s, and allow it by default if the css file modification time is less than a day old

Copying http://www.php.net/styles/theme-base.css
Copying http://www.php.net/styles/theme-medium.css

» wc output/php-chunked-xhtml/**/*.css  
 1884  3784 32631 output/php-chunked-xhtml/styles/03e73060321a0a848018724a6c83de7f-theme-base.css
  606  1242 11091 output/php-chunked-xhtml/styles/03e73060321a0a848018724a6c83de7f-theme-medium.css
 2490  5026 43722 total

< HTTP/2 200 
< server: myracloud
< date: Fri, 21 Oct 2022 12:52:18 GMT
< content-type: text/css
< last-modified: Mon, 17 Oct 2022 12:10:02 GMT
< etag: "634d461a-7f77"
< expires: Sat, 22 Oct 2022 11:06:08 GMT
< cache-control: max-age=86400
< x-cdn: 1

This might just be a wifi or cdn or isp issue, though - see php/web-php#556