[v2] Migration guide updates (#5661)

* Remove yarn examples for consistency with other docs

* Add rename responsive image queries section

* Add postcss plugins section

* Remove checklist (moved to issue 3986)

* Add mixed CommonJS/ES6 section

* Link to Gatsby webpack docs

* Add node id section

* Add section: remove explicit polyfills

* Add section: Change modifyBabelrc to onCreateBabelConfig

* Add section: Change `modifyWebpackConfig` to `onCreateWebpackConfig`

* Add section:  CSS is inlined automatically

* Add introduction and small tweaks

* Correct line numbers

* Improve explanation around movement of libs to peerDependencies

* chore: minor capitalization tweak

* Add edit link

* Split peer dep updates into two sections

* Declarative headings

* Add table of contents

* Use neater Fragment syntax

* More detail on deprecation note

* Inline comments on code examples

* Add Jason's node ID query example

* Use markdown diff syntax for before / after examples

* Add 'in-progress' note about babel polyfills

* Add instructions on how to install Gatsby v2