ci: Check for broken links (#30844)

This PR fixes some broken links that where found using
[lychee](https://github.com/lycheeverse/lychee/) as discussed today with
@JosephTLyons and @nathansobo at the RustNL hackathon. Using
[lychee-action](https://github.com/lycheeverse/lychee-action/) we can
scan for broken links daily to prevent issues in the future.
There are still 6 broken links that I didn't know how to fix myself.
See https://github.com/thomas-zahner/zed/actions/runs/15075808232 for
details.

## Missing images

```
Errors in ./docs/src/channels.md

    [ERROR] file:///home/runner/work/zed/zed/docs/.gitbook/assets/channels-3.png | Cannot find file
    [ERROR] file:///home/runner/work/zed/zed/docs/.gitbook/assets/channels-1.png | Cannot find file
    [ERROR] file:///home/runner/work/zed/zed/docs/.gitbook/assets/channels-2.png | Cannot find file
```

These errors are showing up as missing images on
https://zed.dev/docs/channels
I tried to search the git history to see when or why they were deleted
but didn't find anything.

## ./crates/assistant_tools/src/edit_agent/evals/fixtures/zode/prompt.md

There are three errors in that file. I don't fully understand how these
issues were caused historically. Technically it would be possible to
ignore the files but of course if possible we should address the issues.
 
Release Notes:

- N/A

---------

Co-authored-by: Marshall Bowers <git@maxdeviant.com>
Co-authored-by: Ben Kunkle <ben@zed.dev>

lychee.toml

@@ -0,0 +1,29 @@
+retry_wait_time = 5
+
+accept = ["200..=204", "429"]
+
+max_retries = 5
+
+timeout = 45
+
+exclude = [
+    # Don't fail CI check if collab is down
+    'https://staging-collab.zed.dev/',
+    "https://collab.zed.dev",
+
+    # Slow and unreliable server.
+    'https://repology.org',
+
+    # The following websites are rate limited or use bot detection and aren't nice enough to respond with 429:
+    'https://openai.com',
+    'https://claude.ai/download',
+    'https://www.perplexity.ai',
+    'https://platform.deepseek.com',
+    'https://console.anthropic.com',
+    'https://platform.openai.com',
+    'https://linux.die.net/man/1/sed',
+    'https://allaboutcookies.org',
+    'https://www.gnu.org',
+    'https://auth.mistral.ai',
+    'https://console.mistral.ai',
+]