feat(v4_migration): add first draft of the v4 migration guide (#138)

* First draft on new migration guide and added getting started.

* Edits.

* More edits.

* More edits.

* More edits.

* More edits.

* More edits.

* migration getting started information to getting started guide and plugin configuration page

* Changes on v4 branch

Was on detached head branch. Hopefull this brings it all back to working git goodness!

* Added plugin changes section.

* Add comment about which plugins can be disabled.

* rename "Custom Plugins" page to "Customizing Plugins"

* fix: remove unnecessary title

* chore: copy edits and fix some bugs in the formatting

* Remove todo.

* add more details to the migration guide regarding plugin changes as well as various edits

* fix: add missing configuration options in migration guide

* chore: simplify table entries

* chore: more table simplification for formatting

* chore: add link when referencing other section

* fix: add missing link

* feat: add placeholder for session management recipes

* feat(sessions): add recipes to sessions management page

* feat(customizing_plugins): add walkthrough for how `opts` overriding works

* chore: reorder migration guide

* feat(v4_migration): add utility function migration information

* chore: remove unnecessary todo

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Co-authored-by: Micah Halter <[email protected]>