Update optimizer overview

[som-snytt]

Use the simplified options, but reference the old blog post.

Trim a few ambitious promises to explain more.

[som-snytt]

It would be useful to show a build.sbt snippet for "enable under CI".

Or, mention -opt:none and how to enable during dev workflow.

[som-snytt]

This falls somewhere between "no fair!" and "that's life!"

Checking 1501 external links...
Ran on 1188 files!
- ./_site/overviews/scaladoc/contribute.html

  *  External link https://dotty.epfl.ch/docs/usage/dottydoc.html failed: 404 No error

- ./_site/scala3/reference/other-new-features/explicit-nulls.html
htmlproofer 3.15.3 | Error:  HTML-Proofer found 3 failures!
  *  External link https://dotty.epfl.ch/docs/internals/explicit-nulls.html failed: 404 No error
- ./_site/scala3/scaladoc.html
  *  External link https://dotty.epfl.ch/docs/usage/scaladoc/blog.html failed: 404 No error
Error: Process completed with exit code 1.

[SethTisue]

I've requested a few trivial edits.

Maybe the compiler-options page could link to this new page?

[SethTisue]

@lrytz I suggested to Som that after this is merged, I could take care of adding a notice to the top of your blog post directing people here instead.

[lrytz]

👍 thank you!

[som-snytt]

I caved on Seth's points because I just read in the Sylvia Plath bio that Ted Hughes removed three poems from his book because Marianne Moore asked him to.