Add introduction and about sections for hashes

Author: meatball133

concepts/hashes/about.md

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+# About
+
+## Creating hashes
+
+
+
+## Indexing
+
+## Modifying
+
+## Methods
+
+has_value?
+include?
+
+keys
+values

concepts/hashes/introduction.md

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+# About
+
+[Symbols][symbols] are named identifiers that can be used to refer to a value.
+Symbols are created through a symbol literal, which is by prefixing a name with a `:` character, e.g. `:foo`.
+They also allow for being written with quotes, e.g. `:"foo"`, which allows, for example, spaces in the name.
+
+```ruby
+:foo # => :foo
+:"foo boo" # => :"foo boo"
+```
+
+Symbols are used in many places in the language, including as keys in hashes, to represent method names and variable names.
+
+## Identifier
+
+What makes symbols different from strings is that they are identifiers, and do not represent data or text.
+This means that two symbols with the same name are always the same object.
+
+```ruby
+"foo".object_id # => 60
+"foo".object_id # => 80
+:foo.object_id # => 1086748
+:foo.object_id # => 1086748
+```
+
+## Modifying Symbols
+
+Symbols are immutable, which means that they cannot be modified.
+This means that when you "modify" a symbol, you are actually creating a new symbol.
+There are a few methods that can be used to manipulate symbols, they all return new symbols.
+All methods can be found in the [Symbol API][symbols-api].
+
+```ruby
+:foo.upcase # => :FOO
+
+:foo.object_id # => 1086748
+:foo.upcase.object_id # => 60
+```
+
+The benefit of symbols being immutable is that they are more memory efficient than strings, but also safer to use as identifiers.
+
+## Conversion
+
+Symbols can be converted to strings and vice versa.
+This can be useful when you want to modify a symbol, or when you want to use a symbol as a string.
+To present a string as a symbol, you can use the `String#to_sym` method, and to do the opposite, you can use the `Symbol#to_s` method.
+Due to symbols having a limited set of methods, it can be useful to convert a symbol to a string to use string methods on it, if a new symbol is needed.
+
+```ruby
+:foo.to_s # => "foo"
+"foo".to_sym # => :foo
+```
+
+[symbols]: https://www.rubyguides.com/2018/02/ruby-symbols/
+[symbols-api]: https://rubyapi.org/o/symbol