docs(README): Write the Introduction section

Signed-off-by: 林博仁(Buo-ren Lin) <buo.ren.lin@gmail.com>

Author: brlin-tw

README.md

@@ -7,6 +7,17 @@ A utility to convert the navigation sidebar of GitHub documentation pages from H
 
 \#utility \#github-docs \#markdown
 
+## Introduction
+
+This script converts the GitHub Actions documentation sidebar HTML structure
+to a GitHub Flavored Markdown (GFM) unordered list format.
+
+The script extracts:
+
+* Text content from span elements with class "prc-ActionList-ItemLabel-TmBhn"
+* URL links from href attributes of anchor elements
+* Hierarchical structure based on CSS depth indicators (--subitem-depth)
+
 ## Licensing
 
 Unless otherwise noted([comment headers](https://reuse.software/spec-3.3/#comment-headers)/[REUSE.toml](https://reuse.software/spec-3.3/#reusetoml)), this product is licensed under [the 3.0 version of the Affero General Public License](https://www.gnu.org/licenses/agpl-3.0.html), or any of its more recent versions of your preference.