Add NAT rules limit to VPN gateway FAQ

[lksharath]

Added information about the maximum number of NAT rules on a VPN gateway.

[prmerger-automator[bot]]

@lksharath : Thanks for your contribution! The author(s) and reviewer(s) have been notified to review your proposed change.

[learn-build-service-prod[bot]]

Learn Build status updates of commit 3bf67a3:

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articles/virtual-wan/virtual-wan-faq.md	:white_check_mark:Succeeded

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[ktoliver]

@cherylmc

Can you review the proposed changes?

IMPORTANT: When the changes are ready for publication, adding a #sign-off comment is the best way to signal that the PR is ready for the review team to merge.

#label:"aq-pr-triaged" @MicrosoftDocs/public-repo-pr-review-team