update link usage

[indirectlylit]

This PR changes nearly all instances of internal and external links to use props instead of slots, from this form:

Follow the <DocsExternalLink href="https://material.io/design">
  Material design guidelines
</DocsExternalLink>.

to this form

Follow the <DocsExternalLink text="Material design guidelines" href="https://material.io/design" />.

This has two advantages:

A) the previous form causes undesirable underline behaviors:

before	after

B) The code is more concise.

Note while possibly desirable for consistency with <a href="...">...</a>, it is not possible to write this form:

Follow the <DocsExternalLink href="https://material.io/design">Material design guidelines</DocsExternalLink>.

It appears that Vue-ESLint always inserts additional whitespace, and I can't determine any way of disabling this behavior.

[indirectlylit]

self-merge

[indirectlylit]

also relevant - here is how we eslint from messing up whitespace internally within the component:

kolibri-design-system/docs/common/DocsExternalLink.vue

Lines 3 to 6 in 3e56c3f

	<!-- eslint-disable -->
	<!-- prevent line from wrapping so that icon displays correctly -->
	<a :href="href" target="_blank"><template v-if="text">{{ text }}</template><template v-else><slot></slot></template><KIcon icon="openNewTab" class="pop" disableColor /></a>
	<!-- eslint-enable -->