[infra] Factor out include partial from shortcode

[chalin]

• Refactoring in prep for [i18n] Factor prose out of shortcodes like docs/latest-release #6460 and other work
• Why this change: we can't call shortcodes from other shortcodes or partials (without resorting to .RenderString magic, which doesn't work when we're trying to keep things all in markdown).
• Factors out the core functionality of the includes.html shortcode and makes it available as a partial of the same name
• In prep for 6460, this refactors the docs/languages/index-intro.md shortcode for better separate concerns: have the natural language part in an include file, while keeping the code in a shortcode
  • An include-file version of the docs/languages/index-intro.md shortcode
  • A new version of the shortcode (suffixed with "2"), which isn't currently used

The only change in the generated site files are for pages with includes. Any included text with an apostrophe (') is now encoded as ’:

$ (cd public && g diff -bw --ignore-blank-lines -I "'|’") | grep ^diff | head
diff --git a/site/index.html b/site/index.html
diff --git a/sitemap.xml b/sitemap.xml

Preview: https://deploy-preview-6494--opentelemetry.netlify.app/docs/languages/cpp/instrumentation/

Here's what the diff looks like:

[tiffany76]

RSLGTM, with one non-blocking question.

[tiffany76]

I know this introduction is how it's been done for ages, but I find "This is" a bit weak, like it points out the obvious. Maybe we could say something like "Welcome to the OpenTelemetry (name) documentation...."?

[chalin]

Agreed. Let's tackle that change in a followup PR.