Move the migration and contribution guidelines in separate sections

[NataliaTepluhina]

Description of Problem

Currently, the Migration Guide and Contribute to the Docs section are nested in the sidebar under the first docs section (Guide). Here're are some minor issues caused by this nesting:

1. Navigation: When you click the Migration Guide link from the Docs dropdown menu, the sidebar is not scrolling to the relevant section which could cause confusion. For example, if you're on the top of the Template refs section and then click on the Migration Guide from the Docs dropdown, it's not clear there's a list of relevant sub-sections you can navigate to.
2. Copy: The Docs dropdown is repeatedly using the Guide keyword in the dropdown options.
3. Discoverability: It's hard to locate contribution guidelines.

Proposed Solution

1. Move the migration and contribution guidelines in separate top-level sections with their own sidebar.
2. Update the Docs dropdown with something like the following (see screenshots).

Please check deployed preview here

[NataliaTepluhina]

Not sure this is the best name but we need something to wrap 'detailed changes'

[skirtles-code]

Looks good.