doc: write Go 1.14 release notes

[dmitshur]

Tracking bug for writing the Go 1.14 Release Notes.

The latest state on tip can be viewed at https://tip.golang.org/doc/go1.14.

Previously #17929, #15810, #5929, etc.

[gopherbot]

Change https://golang.org/cl/216997 mentions this issue: doc/go1.14: document overlapping interfaces change (update release notes)

[gopherbot]

Change https://golang.org/cl/217099 mentions this issue: doc: document new hash/maphash package

[gopherbot]

Change https://golang.org/cl/217100 mentions this issue: doc/go1.14: fix minot typo (update release notes)

[gopherbot]

Change https://golang.org/cl/217122 mentions this issue: doc/go1.14: mime: .js files now text/javascript

[gopherbot]

Change https://golang.org/cl/217067 mentions this issue: doc/go1.14: mention that debug/dwarf now supports DWARF 5

[gopherbot]

Change https://golang.org/cl/217123 mentions this issue: doc/go1.14: mention new method encoding/json.Decoder.InputOffset

[gopherbot]

Change https://golang.org/cl/217124 mentions this issue: doc/go1.14: mention new field go/build.Context.Dir