_content/doc: convert go1 release notes to markdown

The first set of release notes is a template.
I converted it as follows:

  1. Run html2md.
  2. That escaped ` and * in the template function calls, so
     I manually removed the escapes.
  3. I made some small tweaks to avoid `<` being double-escaped
     so it rendered as `&lt;`.

The remaining diffs are due to "fancy" quotation marks and
apostrophes, as mentioned in the previous CL.

Change-Id: I085f5d141d63fbb751bffeec4dca760d1fd15464
Reviewed-on: https://go-review.googlesource.com/c/website/+/540295
Run-TryBot: Jonathan Amsterdam <[email protected]>
Reviewed-by: Heschi Kreinick <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>