doc: write Go 1.20 release notes

[dmitshur]

This is the tracking issue for writing the Go 1.20 Release Notes. The version at tip can be viewed at https://tip.golang.org/doc/go1.20.

When the Go 1.20 Release Notes are complete, this should be closed, and a similar issue should be made for the next release. Previous issues are #51400, #47694, #44513, etc.

[gopherbot]

Change https://go.dev/cl/425334 mentions this issue: doc: start draft go1.20 release notes

[gopherbot]

Change https://go.dev/cl/425354 mentions this issue: doc/go1.20: populate initial TODOs via relnote

[gopherbot]

Change https://go.dev/cl/453636 mentions this issue: doc/go1.20: add release notes for PGO

[gopherbot]

Change https://go.dev/cl/453620 mentions this issue: doc/go1.20: add release notes for the linker