Improve example "Forward OwnTracks low-battery warnings to ntfy"

[amotl]

This patch improves the provided tutorial about OwnTracks.

• Provide executable tutorial, in order to let users exercise the recipe more easily.
• Choose ntfy as notification target, because the public service can be used without barriers, and it is also easy to self-host.
• Copy-edit tutorial, add screenshots, and provide alternatives to the default configuration.

The corresponding documentation page can be inspected at [Preview] Forward OwnTracks low-battery warnings to ntfy.

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (68e7084) 47.94% compared to head (67ddb8a) 47.94%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #666 +/- ## ======================================= Coverage 47.94% 47.94% ======================================= Files 81 81 Lines 3917 3917 ======================================= Hits 1878 1878 Misses 2039 2039 ``` | Flag | Coverage Δ | | |---|---|---| | unittests | `47.94% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mqtt-tools#carryforward-flags-in-the-pull-request-comment) to find out more.

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