Fix instructions on how to manually trigger a bundle build

[jdeanwallace]

Resolves https://github.com/tiny-pilot/tinypilot/issues/1548

This PR moves the comments that explains how to trigger a code-based bundle build from .circleci/config.yml to .circleci/continue_config.yml and updates the contributing docs to reference the correct CircleCI config file.

So the correct ways to manually trigger a bundle build is either the:

1. Code-based way

   1. In the CircleCI configuration, change the bundle_build_branch parameter’s default value from master to << pipeline.git.branch >>.
   2. Push your changes as branch to GitHub. CircleCI will now automatically build bundles for all subsequent commits of that branch.
   3. Before eventually merging your feature branch, remember to revert the bundle_build_branch parameter to the original value (i.e., master)

2. CircleCI web UI way

   1. On CircleCI’s overview page for the respective branch, click the “Trigger Pipeline” button.
   2. In the dialog, enter bundle_build_branch as parameter name, and the (exact) name of the branch as parameter value.
   3. Click the “Trigger Pipeline” button in the dialog. CircleCI will now build a bundle for the latest commit of that branch.