doc: do not embed style in release-note HTML files

[tomocy]

When styles are embedded in release note html files, it may also require you to do the same thing in the previous ones to keep the same style across all versions.

The embedded style in go1.6 html seems to cause #33718.

I think that it is better to try not to embed style in release note html files, and write it in stylesheet instead.

[bcmills]

CC @andybons

[andybons]

Let's just do this instead of #33718

[gopherbot]

Change https://golang.org/cl/191217 mentions this issue: doc: remove embed style in release-note HTML files

[gopherbot]

Change https://golang.org/cl/191747 mentions this issue: [release-branch.go1.13] doc: align documents link

[gopherbot]

Change https://golang.org/cl/191748 mentions this issue: [release-branch.go1.12] doc: align documents link