[DOCS] Add dev mode guide (#20102)

_data/sidebar.yml

@@ -1,4 +1,5 @@
 Getting started:
+  - Dev mode: /docs/getting-started/dev-mode
 Install and build:
   - Downloads: /downloads
   - Installing Ethereum: /docs/Building-Ethereum

docs/_getting-started/dev-mode.md

@@ -0,0 +1,47 @@
+---
+title: Dev mode
+---
+
+Geth has a development mode which sets up a single node Ethereum test network with a number of options optimized for developing on local machines. You enable it with the `--dev` argument.
+
+Starting geth in dev mode does the following:
+
+-   Initializes the data directory with a testing genesis block
+-   Sets max peers to 0
+-   Turns off discovery by other nodes
+-   Sets the gas price to 0
+-   Uses the Clique PoA consensus engine with which allows blocks to be mined as-needed without excessive CPU and memory consumption
+-   Uses on-demand block generation, producing blocks when there are transactions waiting to be mined
+
+You can specify a data directory to maintain state between runs using the `--datadir` option, otherwise databases are ephemeral and in-memory:
+
+```shell
+$ mkdir test-chain-dir
+$ geth --dev --datadir test-chain-dir console
+```
+
+Once geth is running in dev mode, you can interact with it in the same way as when geth is running in other ways.
+
+For example, create a test account:
+
+```shell
+> personal.newAccount()
+```
+
+Then transfer ether from the coinbase to the new account:
+
+```shell
+> eth.sendTransaction({from:eth.coinbase, to:eth.accounts[1], value: web3.toWei(0.05, "ether")})
+```
+
+And check the balance of the account:
+
+```shell
+> eth.getBalance(eth.accounts[1])
+```
+
+If you want to test your dapps with a realistic block time use the `--dev.period` option when you start dev mode:
+
+```shell
+geth --dev --dev.period 14 console
+```