Docs: clarify checkout-based development, Bundler usage, and test instructions

[id774]

Motivation

• Make the repository easier to use for local development by documenting how to run from a source checkout and how to verify changes before a release.
• Clarify how bundle install behaves and that the plugins group is optional so developers do not install unnecessary dependencies.
• Avoid over-prescriptive platform guidance about native build tools by suggesting installing them only when a gem reports a need.
• Provide an explicit, reproducible way to run the test suite from a checkout with bundle exec rake.

Description

• Updated README.md to add a "From a checkout" section that shows bundle exec bin/automatic --version and bundle exec rake, explains that bundle install resolves runtime and development dependencies from Gemfile and automatic.gemspec, and notes the plugins bundle group is not installed by default.
• Revised doc/DEPLOYMENT.md to soften the native-extension build tool guidance, add bundle exec rake to the checkout workflow, and explain default bundler behavior and optional plugin dependencies.
• Updated doc/PLUGIN_DEVELOPMENT.md to point plugin authors to the checkout setup in README.md (including bundle install and running the default test suite).
• Adjusted doc/QUICKSTART.md to include a short "From a source checkout" path that references the README setup and uses bundle exec bin/automatic for the same flow.
• Minor documentation wording changes such as rephrasing the vendor/ entry as a legacy placeholder.

Testing

• Ran the default development test task with bundle exec rake in a checkout, and the test suite completed successfully.
• RSpec unit tests (the spec/ suite) were exercised as part of the bundle exec rake run and passed.
• No code behavior was changed, only documentation, so CI configuration and test expectations were unchanged and continue to pass.

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Codex Task