Use the file extension as the language class for
highlight.js appears to support file extensions as language
classes, see: https://github.com/highlightjs/highlight.js#supported-languages

It must be more accurate than the file extension.

Dependencies are specified in the setup.py,
and could be resolved from here.
See:
https://pip.pypa.io/en/stable/user_guide/#requirements-files
and https://caremad.io/posts/2013/07/setup-vs-requirement/

Allow empty title for code snippets. It may
be useful for short code fragments.

The documentation is imported from
testcontainers/testcontainers-java@fbf5092

See https://github.com/testcontainers/testcontainers-java/blob/master/docs/contributing_docs.md#codeincludes
and
https://github.com/testcontainers/testcontainers-java/blob/master/LICENSE

Always use UTF-8 as it is commonly used
for platform-independent source files.

The fix in 2979ea4
was incomplete: see the added tests.

Use the last line indent so that the result can be
un-indented by the caller. It prevents results like this:

```
    /* block 1 */
...
    /* block 2 */
```

turning them into

```
    /* block 1 */
    ...
    /* block 2 */
```

which will be dedented by the caller.