docs: add activation calls to examples

[alexvyber]

All the changes are based on code in tests.

The reason for PR is discussed here: https://discord.com/channels/933657521581858818/1438226762000502824

[tdrz]

Hey @alexvyber thank you for this!

We would like to keep the PostgreSQL experience of letting the user activate each extension explicitly with the same mechanism of running CREATE EXTENSION IF NOT EXISTS ...

Therefore the only change that we would like for now is adding this to our docs (see docs/extensions/extensions.data.ts).