#4941 docs/fees limitation in connectors page

A description of the changes proposed in the pull request:

The changes present in this pull request are referred to the ones submitted at #4936. These changes add a new section in beaxy, blocktane, coinzoom, digifinex, kucoin, and okex markdown specific exchange documentation files present in the documentation/docs/exchanges path.

Tests performed by the developer:

The related changes have been tested to render as expected by using a MarkDown compatible editor/viewer.

Tips for QA testing:

No further testing is needed for this specific pull request.

PR Link: https://github.com/hummingbot/hummingbot/pull/4941