Tidy up a few things in the readme and code.

[PromoFaux]

What does this PR aim to accomplish?:

Dipping my toes back into the water.

• Tidy up a few things in the Readme file
  • Add some more comments to the example compose file
  • Add that setting the container's DNS to be itself is not recommended
• Remove some commended out code from startup script
• Move the webpassword checking section into ensure_basic_configuration
• Ensure that FTL's dns.upstreams config is set (either in environment or config file) hard exit if not.

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2. I have commented my proposed changes within the code and I have tested my changes.
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[PromoFaux]

Test need adjustment.

Ah yes, maybe I will move back to its own function for easier testing.

[rdwebdesign]

This link in the Pi-hole features section is broken.

It is pointing to an old section:

• https://github.com/pi-hole/pi-hole/blob/master/README.md#get-help-or-connect-with-us-on-the-web The section "get-help-or-connect-with-us-on-the-web" was removed in 2017. The new section is: getting-in-touch-with-us

Also, we should use a link to the repo main page (example: https://github.com/pi-hole/pi-hole#getting-in-touch-with-us), instead of the file contents (https://github.com/pi-hole/pi-hole/blob/master/README.md#getting-in-touch-with-us).