[Documentation] Add missing "Implicit Flow vs Authorization Code Flow with PKCE" section to MSAL Browser README

[Copilot]

This PR fixes a missing documentation section in the MSAL Browser README that was referenced in the table of contents but didn't exist in the document.

Issue

The MSAL Browser README.md had:

• A table of contents entry for "Authorization Code vs Implicit" linking to #implicit-flow-vs-authorization-code-flow-with-pkce
• An inline reference in the "About" section pointing to the same anchor
• But no actual section with that heading existed, causing broken links

Solution

Added the missing section "## Implicit Flow vs Authorization Code Flow with PKCE" that includes:

Content Added

• Authorization Code Flow with PKCE explanation: Details about the current OAuth 2.0 flow used by MSAL.js 2.x and its security benefits
• Implicit Flow (Deprecated) explanation: Information about the previous flow used in MSAL.js 1.x and why it's deprecated
• Migration Considerations: Guidance for developers migrating from v1.x to v2.x
• References to additional documentation: Links to migration guides and Microsoft identity platform docs

Section Structure

## Implicit Flow vs Authorization Code Flow with PKCE

### Authorization Code Flow with PKCE
- Enhanced Security
- No Tokens in URLs
- Refresh Token Support
- OIDC Compliance

### Implicit Flow (Deprecated)
- Security concerns outlined
- Reasons for deprecation

### Migration Considerations
- Configuration requirements
- Migration guidance

The section is properly positioned between "Build and Test" and "Framework Wrappers" as indicated by the table of contents order.

Fixes #7347.

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.