Web Billing docs restructure

[MrEdwardo]

This is the first step towards more streamlined documentation around Web Billing, that reflects the feature structure, integration paths and future direction. This PR is mostly dealing with restructuring existing content, rather than adding new content — which can be tackled separately.

The goals of this rework are to:

1. Provide a more logical document structure for developers approaching web billing from the beginning
2. Provide more high level guidance around web billing (new overview.mdx page)
3. Elevate Web Paywall Links as a top level low-code integration option
4. Position our Stripe Billing integration as a secondary "external payments" integration, rather than the default option

The general flow is now structured as follows:

1. (1st section) Overview & integration guides
2. (2nd section) Product & other dashboard configuration
3. (remaining top-level docs) Post-integration management of web subscriptions
4. (3rd section) External payments integrations (including Stripe billing)

URLS and redirects

• Only two existing document URLs have changed, for which redirects are added and previous redirects updated - see redirects.js)
• Two new documents are created

[github-actions]

Preview this PR here: https://dev-docs.revenuecat.com/pr-637/

[antoniobg]

Looks good to me. There are some conflicts, probably related to #636