[Feature] Support .jinja, .jinja2, .j2 file extensions for .md docs files

[stevenayers]

Is this your first time submitting a feature request?

• I have read the expectations for open source contributors
• I have searched the existing issues, and I could not find an existing issue for this feature
• I am requesting a straightforward extension of existing dbt functionality, rather than a Big Idea better suited to a discussion

Describe the feature

At the moment doc blocks are only detected in .md files. IDEs often support richer syntax highlighting for .md.j2 extension, so it would be great if we could extend dbt core to look in files with this extension too.

Describe alternatives you've considered

Using an .md file instead. It works absolutely fine, would just be a nice usability improvement.

Who will this benefit?

Everybody who loves docs! :))))

Are you interested in contributing this feature?

Yep sure

Anything else?

No response

[dbeatty10]

Thanks for proposing this @stevenayers!

This is the Markdown-specific complement to #3484, and I'm going to label this as help_wanted accordingly. A contributor may or may not find it's easiest to implement them both at the same time. Here's the most recent time someone made an attempt at this, which may have give ideas / insight: #3498.

For product refinement

I think we'd want to somehow support a variety of the most popular extensions (.jinja, .jinja2, .j2, etc.) -- see #3484 (comment) for more detail.

I think the following could be a relevant interface for users to opt-in to the relevant file extension(s):

dbt_project.yml

model-paths: [directorypath]
seed-paths: [directorypath]
clean-targets: [directorypath]
template-extensions: [fileextension]

Alternatively, we could choose to take a route similar to copier and only allow configuring a single extension per project: templates_suffix in copier.

Examples

Only allowing a single extension

template-extension: ".j2"

Allowing a list

template-extensions: [".jinja", ".jinja2", ".j2", ".templ", ".etc"]

In both cases, the proposed default would be not recognizing any extensions at all. And configuring a template extension would require all files to use it or not be recognized otherwise. Open to feedback on both of these though!