Add links from Rust and C++ to the common platform backend docs

[tronical]

... and adjust the C++ platform backend docs a little to the current
state.

cc #3208

[tronical]

This is the side-bar for the C++ reference. It's mirroring the "Advanced Topics" and there's a little icon that tries to indicate that it's an external link.

In Rust it's under "Related Topics":

[ogoffart]

Can we have comments in .rst and add a comment mentioning sphinx-doc/sphinx#701 (comment) ?

[tronical]

Yes, good point. If we proceed with this, then I'll add that.

[ogoffart]

To be honnest, i don't think this is required. Because these link are accessible already if one follow "slint documentation" and then gets there.
In fact, i think it's even more confusing on the C++ one because you think you stay in the same website but you don't.

That said, i'm not objecting to this change if you like it this way.

[tronical]

The main issue I have is that in the C++ and Rust reference we have documentation about the backend feature selection, but we don't link to the platform backend does.

I'll try to do that for C++ in cmake.md - that section may need an overhaul anyway.

I'd also love to do it for the Rust crate feature docs, but I don't know how to do the concat dance in Cargo.toml where we document the features ;-)

[tronical]

So I agree with you, I'll make at least the change for C++ to remove those "misleading" links and try to have the links to the backend docs right in cmake.md. Those will still be external, but perhaps less misleading.

[ogoffart]

The main issue I have is that in the C++ and Rust reference we have documentation about the backend feature selection, but we don't link to the platform backend does.

Maybe where we talk about the backend, we should link to the relevant documentation
(so in Cargo.toml for rust)

[ogoffart]

I don't know how to do the concat dance in Cargo.toml where we document the features ;-)

Good point, this won't work :-/

[tronical]

The main issue I have is that in the C++ and Rust reference we have documentation about the backend feature selection, but we don't link to the platform backend does.

Maybe where we talk about the backend, we should link to the relevant documentation (so in Cargo.toml for rust)

Yep. Any idea how to do that?

[tronical]

Maybe document_features!() would support not only feature_label but also something like this:

#[doc = document_features!(ext_link_foo = concat!("https://slint.dev/releases/", env!("CARGO_PKG_VERSION"), "/docs/slint/blah.html"))]

and then in Cargo.toml:

## This is the [Foo](${ext_link_foo}) backend.
backend-foo = []

Is that possible in terms of the nested macro use?

[tronical]

New PR has the sidebar links removed, cmake.md overhauled, and just one link added to the "Related Topics" on the Rust side - as a compromise.

[ogoffart]

#[doc = document_features!(ext_link_foo = concat!("https://slint.dev/releases/", env!("CARGO_PKG_VERSION"), "/docs/slint/blah.html"))]

So be my guest if you want to implement that :-)
There is already support for some template, but it could be something like

 #[doc = document_features!(replacments = {LINK_FOO = concat!("https://slint.dev/releases/", env!("CARGO_PKG_VERSION"), "/docs/slint/blah.html") })]

But honestly, i think this is overdoing it.

Either

1. keep it the way you did it
2. hardcode the link to the latest version (slint.dev/docs/slint/...) doesn't matter if people looking our old docs have the wrong link
3. hardcode the link to the snapshot version, and we replace the links in the repository before publishing a version. (we have other things we need to replace, so we can as well add that)
4. implement a something in document-feature

My preference would be 3. , but i'd accept any other.

[tronical]

But honestly, i think this is overdoing it.

Agreed. So let's go with 5 ;-) : A link under "related topics"