Add Claim rewards section and mention using "private" browser browser or clearing cache if doesn't work

[ltfschoen]

Mention that if when claiming your rewards after signing the transaction the loading icon doesn't appear and the transaction doesn't get processed and you get an error in the browser console like in this issue AstarNetwork/astar-apps#870, then a mitigation measure is to clear their browser cache, or to claim the rewards in a "private" browser window (where permissions have been granted to the polkadot.js extension or another wallet extension to be used in a "private" browser window).