Add postgres exporter docs to website

[ArthurSens]

100% AI generated for now, adds prometheus-community/postgres_exporter#1361 to the website :)

[jan--f]

Not sure why 7b2a05e is needed. I'm not against moving exporter related content under a new exporter focused section but including this here makes thing unnecessarily complex.

Including the postgres docs also seems broken. In the preview the link gives 404.

[ArthurSens]

Addressed the feedback during the docs call :)

Including the postgres docs also seems broken. In the preview the link gives 404.

That is because there's no release tag of the Postgres exporter that includes the /docs folder. I've added the commit 752d5d0 that changes the logic from release tag to SHA just so things are visible, but I'd like to remove that commit before merging here

[ArthurSens]

prometheus-community/postgres_exporter#1361 also needs to include FrontMatter headers