docs(featureflags): add GENERIC_CHART_AXES flag

[novemberde]

SUMMARY

I found that GENERIC_CHART_AXES flag exists via superset config file, but GENERIC_CHART_AXES is not described in the document.(This feature was applied via #17917)
This would be helpful for the superset user reading only documents.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[stephenLYZ]

Thanks for the suggestion! I think the only point we might need to check is whether to put it into stable or in testing. cc @rusackas @villebro

[zhaoyongjie]

@novemberde I think that the feature should still need polishing.

[novemberde]

@zhaoyongjie Thank you for your review.
I've moved GENERIC_CHART_AXES feature to "in testing" block.

[srinify]

Thanks for the contribution!