Update octant plugin installation guide

[mengdie-song]

Update octant plugin installation guide and simplify
the steps to deploy octant and antrea-octant-plugin as a Pod.

[antrea-bot]

Thanks for your PR.
Unit tests and code linters are run automatically every time the PR is updated.
E2e, conformance and network policy tests can only be triggered by a member of the vmware-tanzu organization. Regular contributors to the project should join the org.

The following commands are available:

• /test-e2e: to trigger e2e tests.
• /skip-e2e: to skip e2e tests.
• /test-conformance: to trigger conformance tests.
• /skip-conformance: to skip conformance tests.
• /test-whole-conformance: to trigger all conformance tests on linux.
• /skip-whole-conformance: to skip all conformance tests on linux.
• /test-networkpolicy: to trigger networkpolicy tests.
• /skip-networkpolicy: to skip networkpolicy tests.
• /test-windows-conformance: to trigger windows conformance tests.
• /skip-windows-conformance: to skip windows conformance tests.
• /test-windows-networkpolicy: to trigger windows networkpolicy tests.
• /skip-windows-networkpolicy: to skip windows networkpolicy tests.
• /test-hw-offload: to trigger ovs hardware offload test.
• /skip-hw-offload: to skip ovs hardware offload test.
• /test-all: to trigger all tests (except whole conformance).
• /skip-all: to skip all tests (except whole conformance).

[codecov-io]

Codecov Report

Merging #1473 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1473      +/-   ##
==========================================
- Coverage   56.42%   56.39%   -0.03%     
==========================================
  Files         126      126              
  Lines       11662    11662              
==========================================
- Hits         6580     6577       -3     
- Misses       4451     4453       +2     
- Partials      631      632       +1     

Flag	Coverage Δ
integration-tests	45.78% <ø> (ø)
unit-tests	42.26% <ø> (-0.03%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pkg/apiserver/storage/ram/store.go	77.44% <0.00%> (-1.51%)	⬇️
...ntroller/networkpolicy/networkpolicy_controller.go	71.71% <0.00%> (-0.15%)	⬇️

[antoninbas]

LGTM, thanks for making the change

[mengdie-song]

/test-all

[mengdie-song]

/test-conformance