Add SMPP connector catalog docs

[RDPerera]

Summary

• Adds the SMPP connector (v1.0.0) to the catalog under Messaging: connector-overview.md, setup-guide.md, action-reference.md (Actions), trigger-reference.md (Triggers) and example.md, following the exact-title convention in HOW_TO_ADD_A_CONNECTOR_DOCUMENTATION.md.
• Example page walks through binding an SMSC connection with configurables and sending an SMS with a delivery-receipt request, with six screenshots captured from the real WSO2 Integrator UI (en/static/img/connectors/catalog/messaging/smpp/smpp_screenshots_0N_*.png).
• Registers the connector in en/sidebars.ts (between SMB and Snowflake) and adds its catalog card in catalog/index.mdx (icon/version from Ballerina Central; the package is in the ballerina org, so no vendor, like Email/FTP).

Notes

• Sample project: Add SMPP connector sample integration-samples#169 (smpp_connector_sample), referenced by the example page's "Try it yourself" section.
• Supersedes the legacy-site version in Add SMPP connector catalog docs (5.1.x) #727 (that PR targets 5.1.x; this one follows the catalog layout on wso2-connectors and was built from a fresh branch rather than retargeted).
• Preview: https://docs.dilanp.duckdns.org/connectors-preview/catalog/messaging/smpp/connector-overview

Test plan

• npm run typecheck and npx docusaurus build pass; the only broken-link report is the pre-existing shared develop/create-integrations/create-a-new-integration.md link every example page carries
• All relative links and image paths resolve
• Sample compiles with Ballerina 2201.13.4
• No credentials in pages or screenshots

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