Fix link to Understanding Test Rules from techniques pages (#4075)

This bug specifically affects the link to "Understanding Test Rules for WCAG
Success Criteria" found under Test Rules sections in techniques pages
only (understanding pages do not have this bug). The link has the wrong
base path, causing a 404 when built for GitHub Pages or the W3C site.

This PR fixes it by properly referencing `understanding` relatively from
pages under `techniques`; in the full scope of the build system, this
then ends up replaced with a fully-qualified URL when processed through
`CustomLiquid.ts`.

Example from https://www.w3.org/WAI/WCAG22/Techniques/aria/ARIA4:

```
<a href="/understanding/understanding-act-rules.html">
```

Example of same link built in `publication` mode from this PR:

```
<a href="https://www.w3.org/WAI/WCAG22/Understanding/understanding-act-rules.html">
```

_includes/test-rules.html

@@ -1,6 +1,6 @@
 {% if testRules.size > 0 %}
 {%- if isTechniques -%}
-  {% assign understandingActRulesHref = "/understanding/understanding-act-rules.html" %}
+  {% assign understandingActRulesHref = "../../understanding/understanding-act-rules.html" %}
 {%- else -%}
   {% assign understandingActRulesHref = "understanding-act-rules.html" %}
 {%- endif -%}