Add missing keys in Understanding Dynamic Global Properties

For page:

https://developers.hive.io/tutorials-recipes/understanding-dynamic-global-properties.html

Need to add the following keys:

average_block_size
confidential_hbd_supply
confidential_supply
current_reserve_ratio
max_virtual_bandwidth
next_daily_maintenance_time
target_votes_per_period

In addition to the new items, research the keys that are known but not covered:

available_account_subsidies
content_reward_percent
delegation_return_period
hbd_print_rate
init_hbd_supply
pending_rewarded_vesting_hive
pending_rewarded_vesting_shares
sps_fund_percent
sps_interval_ledger
vesting_reward_percent