Include rust as an externally maintained procedural language

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/15/external-pl.html
Description:

The documentation [1] contains a list of independently maintained procedural
languages. Recently, PL/Rust [2] has reached its version 1.0 [3] and appears
to be stable. 

Major companies have expressed interest in the extension. Kiwi Copple, the
founder of Supabase, has stated his company is researching the language for
his platform [4]. Meanwhile, Amazon's RDS team has already announced support
for PL/Rust as a trusted language extension [5] on their PostgreSQL
instances [6].

Considering the increasing support and stability for PL/Rust, it should be
referenced within Postgresql's docs [1].

[1] https://www.postgresql.org/docs/current/external-pl.html
[2] https://tcdi.github.io/plrust/plrust.html
[3]
https://github.com/tcdi/plrust/pull/277/commits/a10352b2461276cffdc2d6bf1fe0340143203125
[4] https://news.ycombinator.com/item?id=35501065
[5]
https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/PostgreSQL_trusted_language_extension.html
[6]
https://aws.amazon.com/about-aws/whats-new/2023/05/amazon-rds-postgresql-rust/