Prevent BTR overwriting pages that already exist in the output

[agubler]

Type: bug

The following has been addressed in the PR:

• [x] There is a related issue
• [ ] All code has been formatted with prettier as per the readme code style guidelines
• [x] Unit or Functional tests are included in the PR

Description:

Checks if an index.html already exists for BTR paths that are automatically discovered or navigated to when in on-demand mode.

Resolves #271