Update the V5 release content based on feedback from research

[claireashworth]

What

Update the 'Changes to V5' page based on feedback from research. This includes:

• potentially moving the page to the top of the side bar
• making sure all the links are going to the correct version of Frontend
• adding a section on benefits of migrating to V5

Why

These improvements will help users find and use the information they need to migrate to V5.

Who needs to write and review

Tech writer, Developer

Done when

• [x] Agree on the changes to make
• [x] Draft changes
• [x] Team review and approve
• [x] 2i review from another tech writer or content designer

  
  ### Tasks

• [x] Move the page 'Changes to V5' to the top of the side bar
• [ ] https://github.com/alphagov/govuk-frontend-docs/issues/368
• [x] Check all links are going to the correct version of frontend
• [ ] https://github.com/alphagov/govuk-frontend-docs/issues/367
• [x] Add more information on graded browser support to 'changes to V5' and potentially the release notes and add links to guidance
• [ ] https://github.com/alphagov/govuk-frontend-docs/issues/393
• [ ] https://github.com/alphagov/govuk-frontend-docs/issues/376
• [ ] https://github.com/alphagov/govuk-frontend-docs/issues/372
• [ ] https://github.com/alphagov/govuk-frontend-docs/issues/377
• [ ] https://github.com/alphagov/govuk-frontend-docs/issues/370
• [ ] https://github.com/alphagov/govuk-frontend-docs/issues/369
• [ ] https://github.com/alphagov/govuk-frontend-docs/issues/371
• [ ] https://github.com/alphagov/govuk-frontend-docs/issues/373
• [ ] https://github.com/alphagov/govuk-frontend-docs/issues/374
• [ ] https://github.com/alphagov/govuk-frontend-docs/issues/375
• [ ] https://github.com/alphagov/govuk-frontend-docs/issues/378
• [ ] https://github.com/alphagov/govuk-frontend-docs/issues/379
• [ ] https://github.com/alphagov/govuk-frontend-docs/issues/380

[romaricpascal]

All done 🥳