docs: Rewrite Documentation (v4)

[lewis-sqa]

Description

• Initial work to document the new v4 API of Wallet Selector.
• Exported a few more interfaces.
• Moved guides under the core package.

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

Type of change. This type of change is the main reason for the PR.

• FIX - a PR of this type patches a bug.
• FEATURE - a PR of this type introduces a new feature.
• BUILD - a PR of this type introduces build changes.
• CI - a PR of this type introduces CI changes.
• DOCS - a PR of this type introduces DOCS improvement.
• STYLE - a PR of this type introduces style changes.
• REFACTOR - a PR of this type introduces refactoring.
• PERFORMANCE - a PR of this type introduces performance changes.
• TEST - a PR of this type adds more tests.
• CHORE - a PR introduces other changes than the specified above.

Breaking changes

• BREAKING CHANGE - SPECIFY: _______
• NO BREAKING CHANGE - this PR doesn't contain any breaking changes and it's backwards compatible