Upgrade docs to 8.3, 8.4 and then to 9.x

[ryyppy]

Unrelated to version:

• (done)Add open! docs

8.3:

• (done) remove bs. prefix from the attributes
• (done) Allow more character sets in module names (.something.res)
• (done) Fix Js.Array / Js.Array2 (https://github.com/rescript-lang/rescript-compiler/pull/4597/files)

8.4:

• (done) monorepo support feature pinned deps
• (done) introduce a new warning 109: toplevel expression is expected to have type unit It is turned on as warn-error by default. This warning is introduced to avoid partial application errors in a curried language

Upgrade to 9.0:

• (done) Implement light weight syntax for poly-variants
• (done) New external stdlib improvement (https://rescript-lang.org/blog/release-9-0#new-external-stdlib-configuration)
• (done) when -> if (https://rescript-lang.org/blog/release-9-0#when---if)

[ryyppy]

\cc @namenu