Archive migration docs

[ines-toupeira]

This PR archives the outdated migration-related documents and the relocation and renaming of the migration-integrators.md document to the Build section. The changes include:

• Archiving all documents under the migration section except migration-integrators.md
• Moving migration-integrators.md to the Build section and renaming it to integrator-guide.md

The following documents have been moved to the archive folder for internal reference and are no longer accessible publicly:

• validator-registration.md
• validator-activation.md
• node-operators.md
• migration-json-rpc.md
• migration-web-developers.md
• migration-technical-details.md
• faqs.md

TODO:

1. Create a custom 404 page for the archived links for the readers who attempt to access migration pages
2. Implement a redirect link of the migration-integrators.md to integrator-guide.md
3. Update the integrator-guide.md document once PR Update integrators guide #90 is merged to reflect the doc changes

Note: The netlify checks have failed due to broken links referencing migration-integrators.md. Once all the changes are merged and the redirect is implemented, this issue will be resolved

[netlify]

❌ Deploy Preview for developer-center failed. Why did it fail? →

Name	Link
🔨 Latest commit	c697e21
🔍 Latest deploy log	https://app.netlify.com/sites/developer-center/deploys/677ffa3d67ebb20008839aed