Document the assertion API (README reference table and/or a mockly.org page)

## Problem

This repository ships roughly 25 public assertions across five assertion classes, and none of them are documented anywhere.

- The README here is a stub: install instructions, one `HaveAllRequestsCalled()` example, and a pointer to mockly.org.
- mockly.org documents the **core** library. It mentions assertions in passing, but there is no reference for what this package actually offers.

So the only way to discover `WithBodyHavingPropertiesEqualTo`, `HaveCalledInOrder`, `WithResponseHeader`, `BeUnexpected` and the rest is IntelliSense or reading `Shared/HttpMockAssertionExtensions.cs`. Features that were deliberately built (per-mock invocation assertions, request-header assertions, query-param assertions) are effectively invisible.

## Proposal

Either, or both:

1. **A reference table in this README** — grouped by subject (`HttpMock`, `RequestCollection`, `CapturedRequest`, `RequestMockResponseBuilder`), one line per assertion with a short example.
2. **An assertions page on mockly.org** — the Docusaurus site already exists in the core repo, so this would sit naturally alongside the usage and advanced guides, and is more discoverable for people who never visit this repo.

## Notes

- The XML docs on each method are already good, so much of the content can be lifted directly.
- Worth including the small things that are easy to miss: that `ContainRequestFor` returns `ContainedRequestAssertions` so `With*` calls chain, and that `.Which` drops you back to the `CapturedRequest`.
- A short "which package do I install" note (v7 vs v8, and any future flavour) would help too.

Start with the README and Shared/HttpMockAssertionExtensions.cs, using the existing XML docs as the source for the assertion names and examples. Group the roughly 25 assertions by subject and document chaining through ContainedRequestAssertions and .Which; done means the README reference and/or mockly.org page covers the public assertion API and includes the package-version note.

Written by the indexing model from the issue text.