docs, test, fix: Fit Fast_Power to contributing guidelines and fix bug

[endorphin-naixu]

Fit Fast_Power to contributing guidelines. Return NAN when the base is 0 and the exponent is non-positive I noticed that the original test program used "==" to test the result, and when the exponent is negative, it is obviously unreasonable to use "==" to compare two floating-point numbers, so I introduced a value "epsilon = 1e-8" as the error range. Delete the useless output from the original test code and keep only the output of "All tests have successfully passed!".

Description of Change

Checklist

• [x] Added description of change
• [x] Added file name matches File name guidelines
• [x] Added tests and example, test must pass
• [x] Added documentation so that the program is self-explanatory and educational - Doxygen guidelines
• [x] Relevant documentation/comments is changed or added
• [x] PR title follows semantic commit guidelines
• [x] Search previous suggestions before making a new one, as yours may be a duplicate.
• [x] I acknowledge that all my contributions will be made under the project's license.

Notes: Fix issue #2456

[endorphin-naixu]

Please take a look @realstealthninja