Update the doxygen template to match current library repos.

[sarenameas]

These templates help developers get started writing doxygen docs. Please see docs/doxygen_guide.md for more information.

The size estimation method was changed in the spoke library repos:

https://github.com/FreeRTOS/coreMQTT/blob/master/docs/doxygen/pages.dox#L23
https://github.com/FreeRTOS/coreJSON/blob/master/docs/doxygen/pages.dox#L19
https://github.com/aws/device-shadow-for-aws-iot-embedded-sdk/blob/master/docs/doxygen/pages.dox#L22

By submitting this pull request, I confirm that my contribution is made under the terms of the MIT license.

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (master@ab23215). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             master     #1273   +/-   ##
==========================================
  Coverage          ?   100.00%           
==========================================
  Files             ?         5           
  Lines             ?       317           
  Branches          ?        73           
==========================================
  Hits              ?       317           
  Misses            ?         0           
  Partials          ?         0           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ab23215...6ff103f. Read the comment docs.