docs: update docs icons (#12465)

### What problem does this PR solve?

Update icons for docs.
Trailing spaces are auto truncated by the editor, does not affect real
content.

### Type of change

- [x] Documentation Update

docs/contribution/contributing.md

@@ -1,6 +1,9 @@
 ---
 sidebar_position: 1
 slug: /contributing
+sidebar_custom_props: {
+  categoryIcon: LucideBookA
+}
 ---
 
 # Contribution guidelines
@@ -32,7 +35,7 @@ The list below mentions some contributions you can make, but it is not a complet
 1. Fork our GitHub repository.
 2. Clone your fork to your local machine:
 `git clone git@github.com:<yourname>/ragflow.git`
-3. Create a local branch: 
+3. Create a local branch:
 `git checkout -b my-branch`
 4. Provide sufficient information in your commit message
 `git commit -m 'Provide sufficient info in your commit message'`