chore: modernize build instructions

[Kha]

Use cmake --preset, adjust and document parallelism settings

[leanprover-community-mathlib4-bot]

Mathlib CI status (docs):

• ❗ Std/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 6e731b4370000a8e7a5cfb675a7f3d7635d21f58 --onto 806e41151b6eb645e4ed5a40915b94b99f933564. (2024-05-02 12:42:50)
• ❗ Std/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 6e731b4370000a8e7a5cfb675a7f3d7635d21f58 --onto b470eb522bfd68ca96938c23f6a1bce79da8a99f. (2024-05-03 07:57:21)
• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 6e731b4370000a8e7a5cfb675a7f3d7635d21f58 --onto ff37e5d512efcd3981290270a2fc3ecb100bbd0c. (2024-05-23 10:58:47)

[kmill]

While we're editing doc/main/index.md, could we add an explanation to the top for who these instructions are for? At a Lean school I taught at, one of the participants' first inclinations as a hardcore Linux user was to get started with Mathematics in Lean by building Lean from source, which immediately caused issues, so it would be good to at least include a pointer to the quickstart. Here's a draft:

These are instructions to set up a working development environment for those who wish to make changes to Lean itself. It is part of the Development Guide.

We strongly suggest that new users instead follow the Quickstart to get started using Lean, since this sets up an environment that can automatically manage multiple Lean toolchain versions, which is necessary when working within the Lean ecosystem.