[Bug] Update PyPI to point at the `stable` branch for the changelog and not at `main`

It's really hard to find the "real" changelogs right now. We need to make it easy.

Two things:
1. Update links in PyPI
2. Update/add links in the `main` branch to point people towards the `stable` branch

Originally posted by @emmyoop in [#1734](https://github.com/dbt-labs/dbt-adapters/issues/1734#issuecomment-4075173994):

> Changelogs for the adapters live on the `stable` branch as that is the branch we release the minor patches from. They do not live on `main`. See dbt-snowflake changelog [here](https://github.com/dbt-labs/dbt-adapters/blob/stable/dbt-snowflake/CHANGELOG.md).
>
> I would suggest we update PyPI to point at this branch for the changelog and not at main.

Also, delete the content of files like [this](https://github.com/dbt-labs/dbt-adapters/blob/main/dbt-snowflake/CHANGELOG.md), and include a link to the changelog on the `stable` branch instead.