Add "Why Immer is Built In" section

[markerikson]

Because this keeps coming up frequently:

• Why immer inside a starter kit? #5
• Consider adding an option to remove immer #183
• RFC: Make immer optional for createReducer #242
• https://www.reddit.com/r/reactjs/comments/srezxl/old_question_what_do_people_in_the_industry_say/hwtoy5o/
• https://www.reddit.com/r/reactjs/comments/squatd/should_we_be_teaching_redux_in_2022/hwp1oi1/
• https://www.reddit.com/r/reactjs/comments/r12i8v/react_state_management/hypmqtl/?context=3

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[github-actions]

size-limit report 📦

Path	Size
1. entry point: @reduxjs/toolkit (cjs.production.min.js)	14.54 KB (0%)
1. entry point: @reduxjs/toolkit (esm.js)	12.38 KB (0%)
1. entry point: @reduxjs/toolkit/query (cjs.production.min.js)	24.29 KB (0%)
1. entry point: @reduxjs/toolkit/query (esm.js)	19.01 KB (0%)
1. entry point: @reduxjs/toolkit/query/react (cjs.production.min.js)	26.58 KB (0%)
1. entry point: @reduxjs/toolkit/query/react (esm.js)	21.7 KB (0%)
2. entry point: @reduxjs/toolkit (without dependencies) (cjs.production.min.js)	7.33 KB (0%)
2. entry point: @reduxjs/toolkit (without dependencies) (esm.js)	7.28 KB (0%)
2. entry point: @reduxjs/toolkit/query (without dependencies) (cjs.production.min.js)	10.12 KB (0%)
2. entry point: @reduxjs/toolkit/query (without dependencies) (esm.js)	10.55 KB (0%)
2. entry point: @reduxjs/toolkit/query/react (without dependencies) (cjs.production.min.js)	2.8 KB (0%)
2. entry point: @reduxjs/toolkit/query/react (without dependencies) (esm.js)	3.21 KB (0%)
3. createSlice (esm.js)	5.08 KB (0%)
3. createEntityAdapter (esm.js)	6.44 KB (0%)
3. configureStore (esm.js)	5.82 KB (0%)
3. createApi (esm.js)	17.22 KB (0%)
3. createApi (react) (esm.js)	19.93 KB (0%)
3. fetchBaseQuery (esm.js)	11.6 KB (0%)
3. setupListeners (esm.js)	10.46 KB (0%)
3. ApiProvider (esm.js)	18.56 KB (0%)