include program code in markdown

[behoppe]

Issues
@ailiop mentioned it would be nice to have example programs residing in a place where we can include their code in the relevant articles instead of copying and pasting. I verified this is possible, as documented here.

Additional discussion
The solution uses Eleventy's include, which means that the example programs should live somewhere inside the /src/_includes folder of the website repo. I think that presents us with the main design decision -- how exactly to organize our code files in that location.

We might put all our examples into /src/_includes/code. Then we could use an example /src/_includes/code/dummy.c by adding the following within the markdown body of an article

```opening ticks probably specify cilkc
{% include "code/dummy.c" %}
```closing ticks

This would show up as pictured below (in a silly example).

@ailiop shall I set up a folder /src/_includes/code? Or are there other possible configurations we'd like to consider?

[ailiop]

I think this is a fine initial setup, thank you @behoppe.

Some thoughts:

• In some cases, such as nontrivial codes for tutorials or blog posts, it may be beneficial to use page-specific subdirectories with Makefiles etc, to make it easier to review/validate/update the page contents if necessary.
• It could be handy if the include shortcode accepted optional arguments with start/end line numbers. For example, this could facilitate showing the code for some function() but not for a test rig in main(), etc.
• Once we agree on a specific coding style, we can use an appropriate .clang-format config file to make sure our codes are formatted accordingly. Moreover, I wonder if we could have some GitHub CI action do that automatically for all C/C++ files in src/_includes/code/.

[behoppe]

Thanks @ailiop. I think we can add subdirectories as we need them.

I consider this item done (with #148), with these two "cans" getting "kicked":

• include is built into the template language, and it does not support line numbers. Perhaps it's possible to write a separate shortcode that can encapsulate anything (e.g., include) and crop it? I leave that to a separate project.
• I am glad to hear about .clang-format and look forward to how we use it to roll out a standard. Why don't we leave that for another project as well.