Improve Set Up Validator

[0xLucca]

This page addresses some issues in the Set Up a Validator page

[KarimJedda]

This is the full page to review:

• https://github.com/polkadot-developers/polkadot-docs/edit/master/infrastructure/running-a-validator/onboarding-and-offboarding/set-up-validator.md
• Rendered: https://docs.polkadot.com/infrastructure/running-a-validator/onboarding-and-offboarding/set-up-validator/#

[BulatSaif]

Not related to changes in this PR, but general improvements for the set-up-validator.md page:

1. In the Install from Official Releases section:
   Add a step to check the signature after downloading the binary from GitHub.

2. In the Generate Session Keys section:
   There are many ways to generate and inject keys (I know at least 5). Polkadot.js Apps UI and CLI (curl) are the "back-magic" methods. I like KILT instruction it includes manual generation Subkey, which can help explain how the keys are generated and stored. KILT Docs: Generating Session Keys.

[0xLucca]

Not related to changes in this PR, but general improvements for the set-up-validator.md page:

1. In the Install from Official Releases section:
   Add a step to check the signature after downloading the binary from GitHub.
2. In the Generate Session Keys section:
   There are many ways to generate and inject keys (I know at least 5). Polkadot.js Apps UI and CLI (curl) are the "back-magic" methods. I like KILT instruction it includes manual generation Subkey, which can help explain how the keys are generated and stored. KILT Docs: Generating Session Keys.

Thanks @BulatSaif. I'll check your suggestions and add them to the page

[0xLucca]

I will add the checksum verification in this PR. Then, I will create a new one to split this page into multiple ones and add the key management suggested because the page is too long

[eshaben]

LGTM!

[eshaben]

@0xLucca there are some open convos. If everything has been addressed, please update Needs Review label to Ready to Merge