[Portal] Universal Bridge .NET (#6625)

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## PR-Codex overview
This PR focuses on the removal of several payment-related pages in the `.NET SDK` documentation and the addition of a new section for the `Universal Bridge`, including examples and detailed API usage.

### Detailed summary
- Deleted pages:
  - `quickstart/page.mdx`
  - `buywithfiat/page.mdx`
  - `buywithcrypto/page.mdx`
  - `getbuyhistory/page.mdx`
  - `getbuywithfiatquote/page.mdx`
  - `getbuywithfiatstatus/page.mdx`
  - `getbuywithcryptoquote/page.mdx`
  - `getbuywithcryptostatus/page.mdx`
  - `getbuywithfiatcurrencies/page.mdx`
- Updated `sidebar.tsx` to remove payment links and add a new `Bridge` section.
- Introduced `Universal Bridge` documentation with:
  - Metadata creation
  - Core API examples for buying, selling, and transferring tokens.
  - Code snippets for `C#` usage of the `ThirdwebBridge`.

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Author: 0xFirekeeper