Changes for documentation - Batch 2

[yongenaelf]

Content changes:

• [x] https://docs.aelf.com/quick-start/node-operators/
• [x] https://docs.aelf.com/quick-start/node-operators/set-up-a-side-chain/ - move to quick start
• [x] Setup local environment category under Tools section

Links

• [x] https://docs.aelf.com/quick-start/node-operators/set-up-a-node-on-testnet/ - add environment setup link
• [x] https://docs.aelf.com/quick-start/node-operators/set-up-a-node-on-mainnet/ - add environment setup link
• [x] https://docs.aelf.com/quick-start/node-operators/simulating-a-bp-node/ - add contract deployment and dApp development guide links
• [x] https://docs.aelf.com/learn/cross-chain/introduction/ - add links to architecture and setup main chain
• [x] https://docs.aelf.com/tools/chain-sdk/php-sdk/ - fix php link
• [x] contributing guide links for Chain SDK pages
• [x] header/footer links to playground should lead to playground docs
• [x] Update aelf.com website with updated links (planned)

Rewrite descriptions

• [x] https://docs.aelf.com/learn/cross-chain/
• [x] https://docs.aelf.com/tools/smart-contract-templates/
• [x] https://docs.aelf.com/resources/