docs: add dedicated session management guide

[Adii-45]

Which problem is this PR solving?

Closes #114

This PR adds dedicated documentation for browser session management to improve discoverability and provide a more comprehensive reference than the existing SDK README!

Previously, session management was only documented as a brief section inside packages/sdk/README.md, making it difficult for users to understand the available APIs, lifecycle, configuration options, extension points, and implementation details!

Short description of the changes

Added

• A new docs/session-management.md guide covering:
  • Session overview
  • Quick start
  • Session lifecycle
  • SessionManager configuration
  • Built-in implementations
  • Registering session processors
  • Observing session lifecycle
  • Custom SessionStore implementations
  • Custom SessionProvider implementations
  • Known limitations
  • Related references

Updated

• Simplified the Sessions section in packages/sdk/README.md by keeping it focused on quick-start usage and linking to the new dedicated guide for detailed documentation.
• Added a new Documentation section to the repository README.md linking to:
  • Session Management
  • Browser Events
  • Navigation Event

This keeps detailed documentation centralized under the docs/ directory while preserving a concise onboarding experience in the SDK README.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

How Has This Been Tested?

• Verified all Markdown links resolve correctly
• Reviewed the documentation for accuracy against the current implementation
• Confirmed no runtime code or public APIs were modified
• Ran the existing test suite (npm test) to ensure no regressions

Checklist

• I followed the style guidelines of this project
• Unit tests have been added
• Documentation has been updated

[overbalance]

When we pass our own processors here but no exportConfig, the SDK does not add an OTLP exporter (the check in startTracesSdk.ts and startLogsSdk.ts is !config.processors || config.exportConfig). So this Quick Start sets session.id on spans and logs but never exports them. @martinkuba what do you think?

Suggested change

	serviceName: 'my-service',
	serviceName: 'my-service',
	exportConfig: { url: 'https://your-collector.example.com' },

[Adii-45]

@overbalance Just a small bump on this 😄

I've addressed all of the requested documentation changes in the latest commit!

The only remaining discussion is the Quick Start example regarding exportConfig, where I was waiting for guidance since it was tagged for @martinkuba's input.

If you'd prefer me to update the example now, I'm happy to make that change as well. Otherwise, I'll wait for the decision on that discussion!

Whenever you have a chance, I'd appreciate another look. Thanks!