Update 3.0 release notes

And other docs cleanup.

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+title: Legacy Attributes Extension
+
+# Legacy Attributes
+
+## Summary
+
+The Legacy Attributes extension restores Python-Markdown's original attribute
+setting syntax. Older versions of Python Markdown (prior to 3.0) included
+built-in and undocumented support for defining attributes on elements. Most
+users have never made use of the syntax and it has been deprecated in favor of
+[Attribute Lists](attr_list.md). This extension restores the legacy behavior for
+users who have existing documents which use the syntax.
+
+## Syntax
+
+Attributes are defined by including the following within the element you wish to
+assign the attributes to:
+
+```md
+{@key=value}
+```
+
+For example, to define a class to a paragraph:
+
+```md
+A paragraph with the attribute defined {@class=foo}anywhere within.
+```
+
+Which results in the following output:
+
+```html
+<p class="foo">A paragraph with the attribute defined anywhere within.</p>
+```
+
+The same applies for inline elements:
+
+```md
+Some *emphasized{@id=bar}* text.
+```
+
+```html
+<p>Some <em id="bar">emphasized</em> text.</p>
+
+You can also define attributes in images:
+
+```md
+![Alt text{@id=baz}](path/to/image.jpg)
+```
+
+```html
+<p><img alt="Alt text" id="baz" src="path/to/image.jpg" /></p>
+```
+
+## Usage
+
+See [Extensions](index.md) for general extension usage. Use `legacy_attr` as the
+name of the extension.
+
+This extension does not accept any special configuration options.