[#2420] Update User Guide to give more details about the portfolio mode (#2486)

Add PortfolioMode.md to describe portfolio UI differences

Previously, the protfolio mode is only described on `cli.md` and lack details.

Add a new PortfolioMode.md to explain the purpose and UI effects of
the --portfolio flag. I also add related screenshots for documentation.

Author: lihongong

docs/images/reposense-normal-mode-author-commits.png

@@ -220,7 +220,9 @@ credit is given.
 * Default: generates the regular report
 * Alias: `-P` (uppercase P)
 * Example: `--portfolio` or `-P`
+* **Portfolio Mode** simplifies the dashboard for showcasing individual contributions (e.g., personal portfolio websites, resume links).
 
+See [Portfolio Mode UI](portfolioMode.md) for details on the differences.
 <!-- ------------------------------------------------------------------------------------------------------ -->
 
 ### `--repo`, `--repos`, `-r`

docs/images/reposense-normal-mode-author-contributions.png

@@ -0,0 +1,27 @@
+## Portfolio Mode UI
+
+Portfolio Mode is designed to provide a clean and minimalistic overview of a contributor's work. It is intended for showcasing individual contributions in personal portfolios, resumes, or public demos.
+
+### When to use Portfolio Mode?
+Use Portfolio Mode when you want to present:
+- Your contribution timeline and work summary cleanly.
+- A version of the dashboard suited for personal portfolios.
+
+### Key Features:
+| Feature                       | Normal Mode                                                 | Portfolio Mode
+|-------------------------------|-------------------------------------------------------------|--------------------------------|
+| Filters                       | Allow searches, grouping and time period selection          | Simplified filters, non-essential toggles are hidden.             |
+| Visual Focus                  | Code review and auditing.                                   | Visual summary of personal contributions (clean & minimal) |
+| Intended Use                  | Code analysis and detailed review.                          | Personal portfolio showcase, resume link, project website.   |
+
+
+### UI Comparison:
+| Normal Mode (Default)                             | Portfolio Mode (`--portfolio`)                             |
+|---------------------------------------------------|------------------------------------------------------------|
+| ![Normal Mode Contributions Screenshot](../images/reposense-normal-mode-author-contributions.png) | ![Portfolio Mode Contributions Screenshot](../images/reposense-portfolio-mode-author-contributions.png)   |
+| ![Normal Mode Commits Screenshot](../images/reposense-normal-mode-author-commits.png) | ![Portfolio Mode Commits Screenshot](../images/reposense-portfolio-mode-author-commits.png)   |
+
+---
+
+### Notes
+- You can still use other CLI flags (e.g. `--since`, `--until`) in combination with `--portfolio`.