Improve Builder plugin build and documentation

[pchan]

Bug description
The builder plugin usage is confusing and the documentation is not clear.

1. Why do we have 2 different versions of builder ? tanzu builder and a separate builder plugin. The doc section does not mention this.
2. The make targets plugin build does not work. It looks like we have to specifically build builder plugin for things to work.

Expected behavior
Clear documentation on why duplicate functionality exists and how to build .

Steps to reproduce the bug / Relevant debug output

make plugin-build-install-local
/Users/prasadc/dev/repo/tanzu/tanzu-cli/t1/bin/builder plugin build
--path ./cmd/plugin
--binary-artifacts /Users/prasadc/dev/repo/tanzu/tanzu-cli/t1/artifacts/plugins
--version test/e2e/framework/v1.0.0-dev
--ldflags "-X 'github.com/vmware-tanzu/tanzu-plugin-runtime/plugin/buildinfo.Date=2023-07-29' -X 'github.com/vmware-tanzu/tanzu-plugin-runtime/plugin/buildinfo.SHA=c51eea05' -X 'github.com/vmware-tanzu/tanzu-plugin-runtime/plugin/buildinfo.Version=test/e2e/framework/v1.0.0-dev'"
--goflags ""
--os-arch darwin_amd64
--match "*"
--plugin-scope-association-file ./cmd/plugin/plugin-scope-association.yaml
bash: line 1: /Users/prasadc/dev/repo/tanzu/tanzu-cli/t1/bin/builder: No such file or directory
make: *** [plugin-build-darwin-amd64] Error 127

Output of tanzu version

./bin/tanzu version
version: test/e2e/framework/v1.0.0-dev
buildDate: 2023-07-29
sha: c51eea0

Environment where the bug was observed (cloud, OS, etc)
Mac (Darwin)