Add docstrings to url_schemes.py to improve readability (#20545)

Signed-off-by: windsonsea <haifeng.yao@daocloud.io>

docs/mkdocs/hooks/url_schemes.py

 # SPDX-License-Identifier: Apache-2.0
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+"""
+This is basically a port of MyST parser’s external URL resolution mechanism
+(https://myst-parser.readthedocs.io/en/latest/syntax/cross-referencing.html#customising-external-url-resolution)
+to work with MkDocs.
+
+It allows Markdown authors to use GitHub shorthand links like:
+
+  - [Text](gh-issue:123)
+  - <gh-pr:456>
+  - [File](gh-file:path/to/file.py#L10)
+
+These are automatically rewritten into fully qualified GitHub URLs pointing to
+issues, pull requests, files, directories, or projects in the
+`vllm-project/vllm` repository.
+
+The goal is to simplify cross-referencing common GitHub resources
+in project docs.
+"""
+
 import regex as re
 from mkdocs.config.defaults import MkDocsConfig
 from mkdocs.structure.files import Files
@@ -7,11 +26,42 @@ from mkdocs.structure.pages import Page
 
 
 def on_page_markdown(markdown: str, *, page: Page, config: MkDocsConfig,
-                     files: Files):
+                     files: Files) -> str:
+    """
+    Custom MkDocs plugin hook to rewrite special GitHub reference links
+    in Markdown.
+
+    This function scans the given Markdown content for specially formatted
+    GitHub shorthand links, such as:
+      - `[Link text](gh-issue:123)`
+      - `<gh-pr:456>`
+    
+    And rewrites them into fully-qualified GitHub URLs with GitHub icons:
+      - `[:octicons-mark-github-16: Link text](https://github.com/vllm-project/vllm/issues/123)`
+      - `[:octicons-mark-github-16: Pull Request #456](https://github.com/vllm-project/vllm/pull/456)`
+
+    Supported shorthand types:
+      - `gh-issue`
+      - `gh-pr`
+      - `gh-project`
+      - `gh-dir`
+      - `gh-file`
+
+    Args:
+        markdown (str): The raw Markdown content of the page.
+        page (Page): The MkDocs page object being processed.
+        config (MkDocsConfig): The MkDocs site configuration.
+        files (Files): The collection of files in the MkDocs build.
+
+    Returns:
+        str: The updated Markdown content with GitHub shorthand links replaced.
+    """
     gh_icon = ":octicons-mark-github-16:"
     gh_url = "https://github.com"
     repo_url = f"{gh_url}/vllm-project/vllm"
     org_url = f"{gh_url}/orgs/vllm-project"
+
+    # Mapping of shorthand types to their corresponding GitHub base URLs
     urls = {
         "issue": f"{repo_url}/issues",
         "pr": f"{repo_url}/pull",
@@ -19,6 +69,8 @@ def on_page_markdown(markdown: str, *, page: Page, config: MkDocsConfig,
         "dir": f"{repo_url}/tree/main",
         "file": f"{repo_url}/blob/main",
     }
+
+    # Default title prefixes for auto links
     titles = {
         "issue": "Issue #",
         "pr": "Pull Request #",
@@ -27,11 +79,19 @@ def on_page_markdown(markdown: str, *, page: Page, config: MkDocsConfig,
         "file": "",
     }
 
+    # Regular expression to match GitHub shorthand links
     scheme = r"gh-(?P<type>.+?):(?P<path>.+?)(#(?P<fragment>.+?))?"
     inline_link = re.compile(r"\[(?P<title>[^\[]+?)\]\(" + scheme + r"\)")
     auto_link = re.compile(f"<{scheme}>")
 
     def replace_inline_link(match: re.Match) -> str:
+        """
+        Replaces a matched inline-style GitHub shorthand link
+        with a full Markdown link.
+        
+        Example:
+            [My issue](gh-issue:123) → [:octicons-mark-github-16: My issue](https://github.com/vllm-project/vllm/issues/123)
+        """
         url = f'{urls[match.group("type")]}/{match.group("path")}'
         if fragment := match.group("fragment"):
             url += f"#{fragment}"
@@ -39,6 +99,13 @@ def on_page_markdown(markdown: str, *, page: Page, config: MkDocsConfig,
         return f'[{gh_icon} {match.group("title")}]({url})'
 
     def replace_auto_link(match: re.Match) -> str:
+        """
+        Replaces a matched autolink-style GitHub shorthand
+        with a full Markdown link.
+        
+        Example:
+            <gh-pr:456> → [:octicons-mark-github-16: Pull Request #456](https://github.com/vllm-project/vllm/pull/456)
+        """
         type = match.group("type")
         path = match.group("path")
         title = f"{titles[type]}{path}"
@@ -48,6 +115,7 @@ def on_page_markdown(markdown: str, *, page: Page, config: MkDocsConfig,
 
         return f"[{gh_icon} {title}]({url})"
 
+    # Replace both inline and autolinks
     markdown = inline_link.sub(replace_inline_link, markdown)
     markdown = auto_link.sub(replace_auto_link, markdown)