Update billing-limits and Speech Configuration file

[akankshadev99]

This PR addresses an issue in the VAPI documentation where the link to access the billing settings is incorrect. I found this over the docs while navigating through the billing section

The original documentation mentioned the URL:
https://dashboard.vapi.ai/billing
However, this link currently leads to a 404 Not Found page.

The correct URL to access the billing settings is:
https://dashboard.vapi.ai/org/billing

[akankshadev99]

Added an section for concurrency limits as well:
This update to the documentation is made in response to a need for clearer information regarding concurrency limits in Vapi, particularly in the context of the Billing section. Users were encountering confusion about the limits for simultaneous calls and the costs mentioned via the dashboard. The default limit and the process to increase concurrency were not clearly outlined.

Additionally, there was an inquiry in the Discord community from a user regarding the concurrency limits for high volume, which highlighted the need to ensure this information is easily accessible to all users.

[akankshadev99]

Updated the Speech Configuration file with more content, code snippets and examples for better user experience and understanding.

[sahilsuman933]

Thank you for the PR, really appreciate it.