Add missing UPGRADE docs to hex docs, upgrade ex_doc #987

Job	Run time
Format	29s
Build ubuntu stable	24s
Build ubuntu nightly	23s
Build windows stable	1m 8s
Build windows nightly	48s
Build macos stable	58s
Build macos nightly	41s
Clippy ubuntu	29s
OTP 26 / Elixir 1.14 / Rust stable / OS macos	3m 15s
Clippy windows	38s
Clippy macos	47s
OTP 26 / Elixir 1.14 / Rust stable / OS ubuntu	2m 27s
OTP 26 / Elixir 1.14 / Rust nightly / OS macos	3m 24s
OTP 26 / Elixir 1.14 / Rust nightly / OS ubuntu	2m 22s
OTP 25 / Elixir 1.14 / Rust stable / OS macos	3m 54s
OTP 25 / Elixir 1.14 / Rust stable / OS windows	4m 41s
OTP 25 / Elixir 1.14 / Rust stable / OS ubuntu	1m 53s
OTP 25 / Elixir 1.14 / Rust nightly / OS macos	2m 40s
OTP 25 / Elixir 1.14 / Rust nightly / OS windows	3m 51s
OTP 25 / Elixir 1.14 / Rust nightly / OS ubuntu	2m 16s
OTP 24 / Elixir 1.13 / Rust stable / OS macos	2m 52s
OTP 24 / Elixir 1.13 / Rust stable / OS windows	5m 5s
OTP 24 / Elixir 1.13 / Rust stable / OS ubuntu	1m 53s
OTP 24 / Elixir 1.13 / Rust nightly / OS macos	3m 26s
OTP 24 / Elixir 1.13 / Rust nightly / OS windows	3m 51s
OTP 24 / Elixir 1.13 / Rust nightly / OS ubuntu	2m 7s
	56m 42s

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