[CT-2283] Allow index generation for dbt-core off of branches, not just main

### Problem
When there are dbt-docs regressions for previously released versions it is difficult to be able to release just that one fix into `dbt-core` as a new `index.html`. The current workflow is to branch off `dbt-docs` `main` at the last commit that was moved into the `*.latest` branch in `dbt-core` that needs to be backported. Once all `dbt-docs` changes are made, we need to be able to run the index generation on the exact branch and target the right branch in `dbt-core`.

### Solution

Allow branch to be an input for [generate-docs-for-dbt-core.yml](https://github.com/dbt-labs/dbt-docs/blob/main/.github/workflows/generate-docs-for-dbt-core.yml) and the explicitly checkout that branch. Default to `main`.

Also allow an input for the target `dbt-core` branch for the PR. Default to `main`.

Question: If the branch is not `main` should the changelog cleanup still happen on the input branch? Probably.