refactor: Testing, documentation, and docker compose updates.

[jlilja]

This is a follow up PR from https://github.com/ryanwholey/terraform-provider-pihole/pull/70.

Some of the key changes in this PR includes:

• Remove the weekly-test.yml file that was previously introduced in #70. This is now instead combined with the test.yml file which now has the new schedule trigger.
• The test.yml file.
  • Remove old weekly-test.yml file
  • Add the scheduled run to list of triggers.
  • workflow_dispatch trigger added.
  • Has linting fixes applied to it.
  • Has updated workflow name reworded.
  • environment variables has been specifically added to the steps that need them, in order to reduce accidentally exposing that data to other steps.
  • No longer pushes a new json object into docker-compose.yml file. Instead sets a TAG variable and calls the docker compose file. The file will read the TAG variable and perform string interpolation when building the image name + tag.
• Add new docker-run target in makefile which calls docker compose for a development/testing run.
• The readme file.
  • Removes old notice of versions not compatible with current state of this project.
  • Trimming whitespaces added after sentences.
  • Move comments into Github markdown compatible notices for documentation easier on the eyes.
  • List the various testing suites. Add some links to relevant sources of information. Add some notes about local testing.

[jlilja]

I'm not too used to how to work with forks, so the PR lists the commits from the previous PR - not sure why.

Anyhow. I did some changes that are subjective. So, please, if you find any style differences you don't like - LMK! :slightly_smiling_face: