docs: add Bitcall SIP trunking integration guide

[BtDomo]

Description

• Add a Bitcall SIP trunking guide at advanced/sip/bitcall, in the same format as the didlogic and Zadarma guides
• Add Bitcall to the SIP integration navigation, after didlogic
• Document dashboard and API configuration for outbound calls: a byo-sip-trunk credential using username/password authentication on gateway.bitcall.io, plus a byo-phone-number for the caller ID
• State clearly that Bitcall is outbound-only (no DIDs) and that IP-based authentication should not be used, in line with the existing note in the SIP trunking guide about shared Vapi SIP servers

Testing Steps

• Run the app locally using fern docs dev or navigate to preview deployment
• Ensure that the changed pages and code snippets work

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Commit: d7487c4
Status: waiting for staging docs URL