Update README.rdoc [ci skip]

Highlighted code
Update commands.rb content in initialization guide

[ci skip]
Add section for command_tasks in initialization guide

[ci skip]
Change syntax format for example returned values

According to our guideline, we leave 1 space between `#` and `=>`, so we
want `# =>` instead of `#=>`.

Thanks to @fxn for the suggestion.

[ci skip]
Fix RailsServer#start content in initialization guide

[ci skip]
[ci skip] Replace #=> with # =>
Correct guide relating MiddlewareStackProxy with Enumerable

[ci skip] Enumerable.instance_methods &
Rails::Configuration::MiddlewareStackProxy.instance_methods # =>
[:as_json]

MiddlewareStackProxy does not share any notable methods with Enumerable.
Remove outdated guide on using rack builder w/o rails middleware

[ci skip]
Fix broken Wikipedia link.
Update action_controller_overview.md [ci skip]

Code style adherence
The option is called encode_big_decimal_as_string [ci-skip]
syntax error joining tables

syntax error joining tables
syntax error joining/including models

syntax error joining/including models
Fixed typos in shallow routes and routing concerns

Grammatical tense correction: "There exists two" => "There exist two".
Plurality correction: "can be reused inside others resources" => "can be reused inside other resources".
Merge pull request #12942 from kuldeepaggarwal/typos

`skiping` => `skipping` [ci skip]
Merge pull request #12945 from fluxusfrequency/patch-1

Fixed typos in "shallow routes" and "routing concerns" in routing.md [ci skip]
[ci skip] `apropriate` => `appropriate`, spaces added
Revert last two commits mistakenly changing join/include syntax

---

Revert "syntax error joining/including models"

This reverts commit ac35f72f30c47298a20a90c8c537db888b463bbb.

---

Revert "syntax error joining tables"

This reverts commit c365986b48c3e8bc8c7f3fa6a8521616ed5dc138.

---

Comments:

https://github.com/rails/docrails/commit/c365986b48c3e8bc8c7f3fa6a8521616ed5dc138#commitcomment-4630684
Improve reading / style of hashes in AR guide
Merge pull request #12949 from akshay-vishnoi/documentation

[ci skip] `apropriate` => `appropriate`, spaces added