docs: add a guide to building form-associated elements

[AKnassa]

What this does

Adds a guide for building form-associated custom elements — inputs, checkboxes, selects — with FAST.

Why

There were no docs for this. Worse than none, actually: the only page in the repository that covered form association documented a FormAssociated mixin from @microsoft/fast-foundation, a package that no longer exists. So someone following the docs writes class MyInput extends FormAssociated(FASTElement), gets a TypeScript error, and has nowhere to go.

The docs were not merely silent — they were walking people into a wall.

What changed

One new page: sites/website/src/docs/3.x/advanced/form-associated.md.

It documents the approach that actually works today, which a maintainer had already endorsed on the issue: stay on a plain FASTElement, set static formAssociated = true, call attachInternals(), and report your value with setFormValue().

It also covers the parts the four-line recipe leaves out, because each one is a real "why is my form empty?" trap:

• the name attribute — without it your element contributes nothing to FormData at all
• formResetCallback, formStateRestoreCallback and formDisabledCallback
• validity and the built-in error message
• a clear signpost that this replaces the removed FormAssociated mixin, so the old page stops misleading people

How to see it

Preview the docs site and open Advanced → Form-associated elements.

Every TypeScript sample on the page was compiled against the real built fast-element type definitions under the repository's own tsconfig settings, so the code in the guide is code that actually builds.

Deliberately out of scope

The issue thread floats the idea of shipping generic form-associated base classes. That is an API design decision for maintainers, and folding it in here would turn a documentation PR into a design PR. Not touched.

Fixes #7198

[azure-pipelines]

Azure Pipelines:
There may be pipelines that require an authorized user to comment /azp run to run.

[azure-pipelines]

Azure Pipelines:
There may be pipelines that require an authorized user to comment /azp run to run.

[radium-v]

The suggested documentation does not meet the quality expectations for our documentation site.