Remove obsolete or discouraged render examples

Most of these examples no longer work. The ability to render an absolute
path was removed as part of CVE-2016-0752. In Rails 6 we changed `render
file:` to only take exact paths and always to render them as raw files.
Rendering a template with its extensions _is_ supported, but I don't
think should be recommended.

This also removes `render file:` from an example where it isn't
relevant.

[ci skip]

guides/source/layouts_and_rendering.md

@@ -178,17 +178,9 @@ In fact, in the BooksController class, inside of the update action where we want
 render :edit
 render action: :edit
 render "edit"
-render "edit.html.erb"
 render action: "edit"
-render action: "edit.html.erb"
 render "books/edit"
-render "books/edit.html.erb"
 render template: "books/edit"
-render template: "books/edit.html.erb"
-render "/path/to/rails/app/views/books/edit"
-render "/path/to/rails/app/views/books/edit.html.erb"
-render file: "/path/to/rails/app/views/books/edit"
-render file: "/path/to/rails/app/views/books/edit.html.erb"
 ```
 
 Which one you use is really a matter of style and convention, but the rule of thumb is to use the simplest one that makes sense for the code you are writing.
@@ -303,7 +295,7 @@ Calls to the `render` method generally accept five options:
 By default, Rails will serve the results of a rendering operation with the MIME content-type of `text/html` (or `application/json` if you use the `:json` option, or `application/xml` for the `:xml` option.). There are times when you might like to change this, and you can do so by setting the `:content_type` option:
 
 ```ruby
-render file: filename, content_type: "application/rss"
+render template: "feed", content_type: "application/rss"
 ```
 
 ##### The `:layout` Option