docs: fix typos in nav and OPA page title ( Lifecycle Management, Regular Gateway)

[PraveenSoori]

This PR corrects three minor spelling issues visible in the published latest docs:

• Governance/Administer: “Lifecycle Managament” → “Lifecycle Management”
• OPA Validation: “Custom OPA Policy for Regualr Gateway” → “Custom OPA Policy for Regular Gateway”

Changes applied in en/mkdocs.yml (nav labels) and the OPA page H1.
Improves polish and searchability for newcomers.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[tharikaGitHub]

Hi @PraveenSoori,

There is a conflict in this PR. Will you be able to address?

Thanks,
Tharika.

[tharikaGitHub]

Hi @PraveenSoori,

I will close this PR due to inactivity. Please reopen once the concerns are addressed.

Thanks,
Tharika.