some basic views documentation (#34390)

Added a little basic discussion at the beginning of the "views" section of the standard-library documentation, based on [this discourse post](https://discourse.julialang.org/t/could-you-explain-what-are-views/17535/3?u=stevengj) which people seem to have found helpful.

Author: stevengj

doc/src/base/arrays.md

@@ -104,6 +104,18 @@ Base.checkindex
 
 ## Views (SubArrays and other view types)
 
+A “view” is a data structure that acts like an array (it is a subtype of `AbstractArray`), but the underlying data is actually
+part of another array.
+
+For example, if `x` is an array and `v = @view x[1:10]`, then `v` acts like a 10-element array, but its data is actually
+accessing the first 10 elements of `x`. Writing to a view, e.g. `v[3] = 2`, writes directly to the underlying array `x`
+(in this case modifying `x[3]`).
+
+Slicing operations like `x[1:10]` create a copy by default in Julia. `@view x[1:10]` changes it to make a view. The
+`@views` macro can be used on a whole block of code (e.g. `@views function foo() .... end` or `@views begin ... end`)
+to change all the slicing operations in that block to use views.  Sometimes making a copy of the data is faster and
+sometimes using a view is faster, as described in the [performance tips](@ref man-performance-views).
+
 ```@docs
 Base.view
 Base.@view

doc/src/manual/performance-tips.md

@@ -1013,7 +1013,7 @@ example, but in many contexts it is more convenient to just sprinkle
 some dots in your expressions rather than defining a separate function
 for each vectorized operation.)
 
-## Consider using views for slices
+## [Consider using views for slices](@id man-performance-views)
 
 In Julia, an array "slice" expression like `array[1:5, :]` creates
 a copy of that data (except on the left-hand side of an assignment,