channeltypepreference attribute for semver template; providing hyperlinks to example sections

[grokspawn]

Introducing a new semver template attribute which provides authors the capability to deterministically select generated major or minor channels as default when they are for all other measures "equally stable".

This attribute is defaulted to minor-version channels if omitted.

Changes visible in the Catalog Templates page.

[grokspawn]

I opened issue #297 to resolve the new 404s on the advanced topics page causing failures in the linter, so we don't lose track of them. They are not material to these changes, however.