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

[emmyoop]

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 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.