Add source URL to example manifest (#5703)

### What

- Fixes #5701 
- Follow-up to #5699 

### Checklist
* [x] I have read and agree to [Contributor
Guide](https://github.com/rerun-io/rerun/blob/main/CONTRIBUTING.md) and
the [Code of
Conduct](https://github.com/rerun-io/rerun/blob/main/CODE_OF_CONDUCT.md)
* [x] I've included a screenshot or gif (if applicable)
* [x] I have tested the web demo (if applicable):
* Using newly built examples:
[app.rerun.io](https://app.rerun.io/pr/5703/index.html) - **LINKS SHOULD
WORK _ONLY_ HERE**
* Using examples from latest `main` build:
[app.rerun.io](https://app.rerun.io/pr/5703/index.html?manifest_url=https://app.rerun.io/version/main/examples_manifest.json)
* Using full set of examples from `nightly` build:
[app.rerun.io](https://app.rerun.io/pr/5703/index.html?manifest_url=https://app.rerun.io/version/nightly/examples_manifest.json)
* [x] The PR title and labels are set such as to maximize their
usefulness for the next release's CHANGELOG
* [x] If applicable, add a new check to the [release
checklist](https://github.com/rerun-io/rerun/blob/main/tests/python/release_checklist)!

- [PR Build Summary](https://build.rerun.io/pr/5703)
- [Docs
preview](https://rerun.io/preview/0530608694da108802e9898c85ea22905aadb3c5/docs)
<!--DOCS-PREVIEW-->
- [Examples
preview](https://rerun.io/preview/0530608694da108802e9898c85ea22905aadb3c5/examples)
<!--EXAMPLES-PREVIEW-->
- [Recent benchmark results](https://build.rerun.io/graphs/crates.html)
- [Wasm size tracking](https://build.rerun.io/graphs/sizes.html)

crates/re_build_examples/src/manifest.rs

@@ -1,7 +1,8 @@
+use std::path::PathBuf;
+
 use re_build_tools::Environment;
 
 use crate::{Channel, Example};
-use std::path::PathBuf;
 
 /// Collect examples in the repository and produce a manifest file.
 ///
@@ -36,12 +37,14 @@ impl Manifest {
             }
         };
 
+        let base_source_url = get_base_source_url(build_env)?;
+
         let workspace_root = re_build_tools::cargo_metadata()?.workspace_root;
         let manifest = self
             .channel
             .examples(workspace_root)?
             .into_iter()
-            .map(|example| ManifestEntry::new(example, &base_url))
+            .map(|example| ManifestEntry::new(example, &base_url, &base_source_url))
             .collect::<Vec<_>>();
 
         if manifest.is_empty() {
@@ -62,10 +65,11 @@ struct ManifestEntry {
     tags: Vec<String>,
     rrd_url: String,
     thumbnail: Thumbnail,
+    source_url: String,
 }
 
 impl ManifestEntry {
-    fn new(example: Example, base_url: &str) -> Self {
+    fn new(example: Example, base_url: &str, base_source_url: &str) -> Self {
         let name = example.name;
         Self {
             title: example.title,
@@ -77,6 +81,11 @@ impl ManifestEntry {
                 width: example.thumbnail_dimensions[0],
                 height: example.thumbnail_dimensions[1],
             },
+            source_url: format!(
+                "{base_source_url}/examples/{}/{name}/{}",
+                example.language.examples_dir().to_string_lossy(),
+                example.language.entrypoint_path().to_string_lossy()
+            ),
             name,
         }
     }
@@ -124,6 +133,17 @@ fn get_base_url(build_env: Environment) -> anyhow::Result<String> {
     Ok(format!("https://app.rerun.io/commit/{sha}"))
 }
 
+fn get_base_source_url(build_env: Environment) -> anyhow::Result<String> {
+    if build_env == Environment::DeveloperInWorkspace {
+        // There is a high chance the current commit isn't pushed to the remote, so we use main
+        // instead.
+        Ok("https://github.com/rerun-io/rerun/blob/main".to_owned())
+    } else {
+        let commit = re_build_tools::git_commit_short_hash()?;
+        Ok(format!("https://github.com/rerun-io/rerun/tree/{commit}"))
+    }
+}
+
 fn parse_release_version(branch: &str) -> Option<&str> {
     // release-\d+.\d+.\d+(-alpha.\d+)?
 

crates/re_viewer/src/ui/welcome_screen/mod.rs

@@ -35,7 +35,7 @@ impl WelcomeScreen {
                     inner_margin: egui::Margin {
                         left: 40.0,
                         right: 40.0,
-                        top: 32.0,
+                        top: 50.0,
                         bottom: 8.0,
                     },
                     ..Default::default()

crates/re_viewer/src/ui/welcome_screen/welcome_section.rs

@@ -21,7 +21,7 @@ pub(super) fn welcome_section_ui(ui: &mut egui::Ui) {
             .wrap(true),
         );
 
-        ui.add_space(29.0);
+        ui.add_space(18.0);
 
         let bullet_text = |ui: &mut Ui, text: &str| {
             ui.horizontal(|ui| {