User guide: module definition: add 'proxy' attribute

[deining]

#992 addressed how to deal with proxies, so it's a good thing to extend the documentation in order to alleviate the use of a proxy.
This PR closes #993.

[LisaFC]

This is great! Should we add something in the "Add theme module configuration..." intro section linking to the Hugo docs so users will know if/when they might need to change things? Something like:

You can find details of what these configuration settings do in the Hugo modules documentation. Depending on your environment you may need to tweak them slightly, for example by adding a proxy to use when downloading remote modules.

Also is the proxy issue likely to show up for users who have copied the example site instead, in which case we should probably put a note in those docs too.

I can update it myself in this PR or a subsequent one?

[deining]

This is great!

Glad you like it!

Should we add something in the "Add theme module configuration..." intro section linking to the Hugo docs so users will know if/when they might need to change things? Something like:

You can find details of what these configuration settings do in the Hugo modules documentation. Depending on your environment you may need to tweak them slightly, for example by adding a proxy to use when downloading remote modules.

Yes, good idea. Wording sounds clear to me.

Also is the proxy issue likely to show up for users who have copied the example site instead, in which case we should probably put a note in those docs too.

Yes. And we should add proxy = "direct" to the config.toml used by the example site. I will author a PR for this shortly.

I can update it myself in this PR or a subsequent one?

Yes, go ahead. I'm glad if you update my exisiting PR with your useful suggestions. Looking forward to get this PR merged!

[LisaFC]

Excellent!