Clarify default new-tab command in help

[nightt5879]

Summary of the Pull Request

Clarifies the top-level wt help output for the implicit new-tab behavior. When no command is specified, wt treats the command line as new-tab; the help text now states that and points users to wt new-tab --help for options such as -d,--startingDirectory.

References and Relevant Issues

Fixes #9585

Detailed Description of the Pull Request / Additional comments

This adds a CLI11 footer to the root command-line parser, backed by a new CmdHelpFooter resource string. It also updates the existing simple help test to assert that the top-level help mentions new-tab, wt new-tab --help, and -d,--startingDirectory.

Validation Steps Performed

• Parsed src/cascadia/TerminalApp/Resources/en-US/Resources.resw as XML successfully.
• Reviewed the staged diff to confirm the change is limited to command-line help text and the corresponding test assertion.
• Full local C++/TAEF tests were not run because msbuild is not available on this machine's PATH.

PR Checklist

• Closes the cli parameter -d is not shown in help #9585
• Tests added/passed
• Documentation updated
  • If checked, please file a pull request on our docs repo and link it here: #xxx
• Schema updated (if necessary)

[nightt5879]

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