All the job can be done at class level so we can avoid some object
allocation

[ci skip] Modifies the initialization guide to make the initializers process clearer

* tree: (22 commits)
  remove dead code
  add some convenient methods for avoiding array allocations
  hide join_constraints inside the JoinDependency object
  speed up match?
  expose the root node and call it
  just skip the join if it's already there
  speed up finding existing nodes
  make node search more efficient
  remove == so we can see where walking up parents occurs
  push parent up to the superclass
  convert JoinBase to a tree and remove the Node class
  names are guaranteed to be symbols
  eliminate function that is only used in one place
  we will always detect the same node, so just pass the node in
  we do not need to to_s the name all the time
  parent is guaranteed to be the same
  reduce number of comparisons and array allocations
  rename the variable to make more sense with a tree
  walk the tree rather than generating a hash and walking it
  do not convert the tree to a list just for the first node
  ...

Remove the annoying `find_or_build` stuff.

The node must be a child of the parent passed in, so only search
children of the parent node

Rather than search every node in the tree, comparing that node and all
of its parents every time, start at the root from both sides and work
our way down the tree

Modifies the initialization guide to make the initializers process clearer and remove a TODO. [ci skip]

We always want a linked list back to the root node.

viewpathtests file moved

These test were not running at all,
Now it will run as a tests part.

[Validations Guide] Add full stop to the end of sentence [ci skip]

Few fixes which includes revert fixes 

Minor align BR tags with HTML void - with reference to the generators code change - https://github.com/rails/rails/commit/f038d4cc5fe1ac21f92c7e32c61ff85a0c816f3f

original commit 89984419

Reverted here ec8ef1e1

Fixing the typo which is formed a not required link.  Check
here http://api.rubyonrails.org/classes/ActionController/Base.html
under paramters section
keeping it under tt tag

gets reverted here ec8ef1e1

[ci skip] Add explanation on Rack::Sendfile middleware in rails_on_rack.md.

Expose MiddlewareStack#unshift to environment configuration

Fixing repond_with working directly on the options hash