[docs] Refactor "decoders" section to "integrations"

[amotl]

• Wrap previous "decoders" section into toplevel "integrations".
• Improve layout of corresponding "integrations" overview page.

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (01d8d24) 53.40% compared to head (1d166b8) 53.40%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #145   +/-   ##
=======================================
  Coverage   53.40%   53.40%           
=======================================
  Files         101      101           
  Lines        5307     5307           
=======================================
  Hits         2834     2834           
  Misses       2473     2473           

Flag	Coverage Δ
unittests	53.40% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
kotori/daq/decoder/airrohr.py	100.00% <ø> (ø)
kotori/daq/decoder/tasmota.py	100.00% <ø> (ø)
kotori/daq/decoder/tts_ttn.py	89.47% <ø> (ø)

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