Cross reference to equations by number

[super-structure]

Similar to how a <xref> to a table or figure will pull the generated number and/or title (based on an input parameter), a cross reference to an <equation-block> containing an <equation-number> should also pull the number content.

The resulting cross-reference text should consist of a localized variable text "Equation" (or abbreviation "eqn." if preferred). This could be controlled with a parameter: args.equationlink.style. This is similar to tables and figures, but there aren't titles for equation block elements.

Example of Empty Equation Number

An empty cross-reference to the following example, input as <xref href="topic.dita#topic-id/eq-001"/>:

<equation-block id="eq-001">
  <equation-number/>
  <image keyref="equation-image-01">
    <alt>a squared plus b squared.</alt>
  </image>
</equation-block>

should render as "Equation 1" with a hyperlink to the equation number.

Example of Populated Equation Number

<equation-block id="eq-3.2a">
  <equation-number>3.2a</equation-number>
  <image keyref="equation-image-01">
    <alt>a squared plus b squared.</alt>
  </image>
</equation-block>

should render as "Equation 3.2a" with a hyperlink to the equation number.