doc: improve release notes

[jba]

This issue covers a set of improvements to the Go release notes:

• Convert existing release notes from HTML to Markdown.
• Change definition lists to level 4 headings.
• Check that there is a valid release note fragment for each file in api/next.
• Write a command that prints a report for incomplete release note fragments.
• Have that command check that there is a release note for an issue if there is a commit that mentions the issue.
• Write a command to merge release note fragments into a single file.
• Handle standard library symbol references in release note fragments like the go/doc/comment package would.

[gopherbot]

Change https://go.dev/cl/541975 mentions this issue: _content/doc: remove html release notes

[gopherbot]

Change https://go.dev/cl/541976 mentions this issue: _content/doc: change relnote definition lists to headings

[gopherbot]

Change https://go.dev/cl/542495 mentions this issue: relnote: a package for working with release notes

[gopherbot]

Change https://go.dev/cl/542515 mentions this issue: doc/next: directory for release note fragments

[gopherbot]

Change https://go.dev/cl/542516 mentions this issue: cmd/relnote,doc/next: add release note check

[gopherbot]

Change https://go.dev/cl/542615 mentions this issue: Add paragraph to 1.8 relnotes.