How to add TLS CA to browser's trust stores

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What does this PR aim to accomplish?:

Add documentation about how to trust TLS/SSL certificate automatically generated by Pi-hole in both Firefox and Chrome.

How does this PR accomplish the above?:

Write text, add pictures

Link documentation PRs if any are needed to support this PR:

This is the docs PR.

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[DL6ER]

Squashed all commits into one

[github-actions]

Images automagically compressed by Calibre's image-actions ✨

Compression reduced images by 8.4%, saving 21.24 KB.

Filename	Before	After	Improvement	Visual comparison
docs/images/api/android-chrome-trusted.png	132.67 KB	122.86 KB	-7.4%	View diff
docs/images/api/android-chrome-untrusted.png	120.74 KB	109.31 KB	-9.5%	View diff

82 images did not require optimisation.

Update required: Update image-actions configuration to the latest version before 1/1/21. See README for instructions.