Update links in the documentation (#5175)

- Where applicable, use a regular [`reference`] instead of
`../../../reference/index.html`.
- Typos.
- Update a link to `polkadot-evm` which has moved out of the monorepo.
- ~~The link specification for `chain_spec_builder` is invalid~~
(actually it was valid) - it works fine without it.

Part of https://github.com/paritytech/eng-automation/issues/10