docs: update the help doc for --oci-layout-path

[Wwwsylvia]

What this PR does / why we need it:

1. Use lowercase letters after feature flags in help docs
2. Align the help docs for other commands with feature flags
3. Add E2E tests for the feature flag of --oci-layout-path for core commands

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #1607

Please check the following list:

• Does the affected code have corresponding tests, e.g. unit test, E2E test?
• Does this change require a documentation update?
• Does this introduce breaking changes that would require an announcement or bumping the major version?
• Do all new files have an appropriate license header?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.30%. Comparing base (ad8c845) to head (c27a8aa).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1609   +/-   ##
=======================================
  Coverage   84.30%   84.30%           
=======================================
  Files         120      120           
  Lines        5449     5449           
=======================================
  Hits         4594     4594           
  Misses        608      608           
  Partials      247      247           

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[Wwwsylvia]

oras push -h BEFORE:

oras push -h AFTER:

[shizhMSFT]

LGTM

[TerryHowe]

/lgtm