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: c51eea05

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