Link to the merged proposal instead of the pull request

Description of the bug:

While reading the design documentation I noticed that there are a lot of links to proposals. All of these links go directly to the pull request of the proposal. Since it is somewhat difficult to go from the pull request to the merged proposal Markdown file but trivial to go from the Markdown file to the pull request (there is a link at the top), I propose to change these links to point to the merged proposal Markdown file instead.

Link to documentation:

https://github.com/carbon-language/carbon-lang/tree/trunk/docs/design#string-types

currently says:

Proposal #820: Implicit conversions

What should it say instead?

Proposal #820: Implicit conversions