docs: fix some broken links (#22167)

This fixes broken links that where reported via:

https://validator.w3.org/checklink?uri=http%3A%2F%2Fgeth.ethereum.org%2Fdocs%2F&hide_type=all&recursive=on&depth=10&check=Check

It also moves the 'getting started' pages from /getting-started/... to
/docs/getting-started/... In order to not break any old links, redirects
from the old location have been added.

Co-authored-by: Felix Lange <fjl@twurst.com>

docs/_dapp/mobile-accounts.md

@@ -48,7 +48,7 @@ Cryptography](sec2), implemented by the [`libsecp256k`](secp256k1) library and w
 [`github.com/ethereum/go-ethereum/accounts`](accounts-go). Accounts are stored on disk in
 the [Web3 Secret Storage](secstore) format.*
 
-[sec2]: http://www.secg.org/sec2-v2.pdf
+[sec2]: https://www.secg.org/sec2-v2.pdf
 [accounts-go]: https://godoc.org/github.com/ethereum/go-ethereum/accounts
 [secp256k1]: https://github.com/bitcoin-core/secp256k1
 [secstore]: https://github.com/ethereum/wiki/wiki/Web3-Secret-Storage-Definition

docs/_dapp/native-accounts.md

@@ -45,7 +45,7 @@ environments.
 
 *For those interested in the cryptographic and/or implementation details, the key-store
 uses the `secp256k1` elliptic curve as defined in the [Standards for Efficient
-Cryptography](http://www.secg.org/sec2-v2.pdf), implemented by the [`libsecp256k`](https://github.com/bitcoin-core/secp256k1) library and wrapped by
+Cryptography](https://www.secg.org/sec2-v2.pdf), implemented by the [`libsecp256k`](https://github.com/bitcoin-core/secp256k1) library and wrapped by
 [`github.com/ethereum/go-ethereum/accounts`](https://godoc.org/github.com/ethereum/go-ethereum/accounts). Accounts are stored on disk in
 the [Web3 Secret Storage](https://github.com/ethereum/wiki/wiki/Web3-Secret-Storage-Definition) format.*