Update docs

[b-per]

This PR adds small update to the docs but has a big diff due to many files getting renamed in the process.

This

• fixes some of the examples that were not showing (fixed by providing generate --provider-name dbt to tfplugindocs)
• updated a few other examples that were not working or just missing
• changed dbt to lowercase everywhere it was still DBT 😄
• moved the example for postgres_credential to the example folder
• mentions the fact we can use env vars on the main page

[b-per]

Comment on the version number

Thanks for the update on the version number. Right now in the end it is still useful when doing a make install and push changes to a specific version but I will keep in mind that it is not used otherwise.

Comment on this PR

About this PR, I have second thoughts about using just dbt when generating the docs. I was eavesdropping other providers and most (if not all of them) use single words. The ones being "multiple words" just concatenate them into one. For example ionoscloud or octopusdeploy

So, I think that the docs could/should be generated with generate --provider-name dbtcloud (instead of the current PR changing it to generate --provider-name dbt)