docs(github): add JSDoc comments for achievement utilities (JhaSourav07#929)

## Description

Added JSDoc documentation for `generateAchievements` and the milestone
constants in `lib/github.ts.`

### Changes made

- Added documentation for `generateAchievements`
- Added documentation for `CONTRIBUTION_MILESTONES`
- Added documentation for `STREAK_MILESTONES`
- Added `@param`, `@returns`, and `@example tags`
- Documented progress calculation and clamping behavior
- No functional changes

Fixes JhaSourav07#350

## Pillar

- [ ] 🎨 Pillar 1 — New Theme Design
- [ ] 📐 Pillar 2 — Geometric SVG Improvement
- [ ] 🕐 Pillar 3 — Timezone Logic Optimization
- [x] 🛠️ Other (Bug fix, refactoring, docs)

## Visual Preview
<img width="1171" height="562" alt="image"
src="https://github.com/user-attachments/assets/03d9f25d-b982-4578-b62d-0d02d88606cc"
/>

<img width="1257" height="725" alt="image"
src="https://github.com/user-attachments/assets/f0740825-c944-4231-be3c-b7ef4784c567"
/>


## Checklist before requesting a review:

- [x] I have read the `CONTRIBUTING.md` file.
- [x] I have tested these changes locally
(`localhost:3000/api/streak?user=YOUR_USERNAME`).
- [x] I have run `npm run format` and `npm run lint` locally and
resolved all errors (CI will fail otherwise).
- [x] My commits follow the Conventional Commits format (e.g.,
`feat(themes): ...`, `fix(calculate): ...`).
- [x] I have updated `README.md` if I added a new theme or URL
parameter.
- [x] I have started the repo.
- [x] I have made sure that i have only one commit to merge in this PR.
- [x] The SVG output matches the CommitPulse "premium quality" aesthetic
standard (no raw elements, smooth animations, correct fonts).
- [ ] (Recommended) I joined the CommitPulse Discord community for
contributor discussions, mentorship, and faster PR support.

Author: JhaSourav07

lib/github.ts

@@ -496,6 +496,30 @@ export async function getWrappedData(username: string, year: string) {
  * UTILS & EXPORTS
  * ========================================================================== */
 
+/**
+ * Generates contribution and streak achievement metadata for a user.
+ *
+ * Achievements are unlocked when the provided contribution count
+ * or streak value meets the corresponding milestone threshold.
+ *
+ * Progress values are clamped between 0 and 100 using `Math.min`
+ * to prevent percentages from exceeding 100%.
+ *
+ * @param totalContributions - Total number of contributions made by the user.
+ * @param currentStreak - Current active contribution streak in days.
+ * @param weekendCommits - Total number of contributions made on weekends.
+ * @param uniqueLanguages - Number of distinct languages used.
+ *
+ * @returns An array of achievement objects for contribution and streak milestones,
+ * including unlock status, progress percentage, threshold values, and display metadata.
+ *
+ * @example
+ * ```ts
+ * const achievements = generateAchievements(125, 10);
+ *
+ * console.log(achievements);
+ * ```
+ */
 export function generateAchievements(
   totalContributions: number,
   currentStreak: number,