Issue:
Bug: docs: API-ref revamp for better user experience



API-ref revamp for better user experience

Pull request description:
## Type of Change
<!-- Put an `x` in the boxes that apply -->

- [ ] Bugfix
- [ ] New feature
- [ ] Enhancement
- [ ] Refactoring
- [ ] Dependency updates
- [X] Documentation
- [ ] CI/CD

## Description
<!-- Describe your changes in detail -->

We have changes the Intro page of the API-ref to make it more informative and navigable.
Also, The API sidebar is now re-organised basis of different use-cases to improve the dev experience.
Added an introduction page to payments API explaining all the scenarios an user can explore payment API. 
### Additional Changes

- [ ] This PR modifies the API contract
- [ ] This PR modifies the database schema
- [ ] This PR modifies application configuration/environment variables

<!--
Provide links to the files with corresponding changes.

Following are the paths where you can find config files:
1. `config`
2. `crates/router/src/configs`
3. `loadtest/config`
-->


## Motivation and Context
<!--
Why is this change required? What problem does it solve?
If it fixes an open issue, please link to the issue here.

If you don't have an issue, we'd recommend starting with one first so the PR
can focus on the implementation (unless it is an obvious bug or documentation fix
that will have little conversation).
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Closes #7323 
## How did you test it?
<!--
Did you write an integration/unit/API test to verify the code changes?
Or did you test this change manually (provide relevant screenshots)?
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<img width="1140" alt="Screenshot 2025-02-20 at 5 55 09 PM" src="https://github.com/user-attachments/assets/9e979280-e081-48a0-b60e-33135d2461ec" />
<img width="1153" alt="Screenshot 2025-02-20 at 5 56 20 PM" src="https://github.com/user-attachments/assets/52f6adb1-3d5d-4ba7-99c9-3fdbadb2d4d4" />


## Checklist
<!-- Put an `x` in the boxes that apply -->

- [ ] I formatted the code `cargo +nightly fmt --all`
- [ ] I addressed lints thrown by `cargo clippy`
- [ ] I reviewed the submitted code
- [ ] I added unit tests for my changes where possible


Testing:

<img width="1140" alt="Screenshot 2025-02-20 at 5 55 09 PM" src="https://github.com/user-attachments/assets/9e979280-e081-48a0-b60e-33135d2461ec" />
<img width="1153" alt="Screenshot 2025-02-20 at 5 56 20 PM" src="https://github.com/user-attachments/assets/52f6adb1-3d5d-4ba7-99c9-3fdbadb2d4d4" />

Patch: