[docs] Missing warning redirects

Nobody has claimed this yet.

Following warning kinds do not have redirects:

- 'reactive-property-without-getter': https://github.com/lit/lit/blob/462350126d51d56f9211d50db43762d81a8b8ed1/packages/reactive-element/src/reactive-element.ts#L743
- 'no-override-create-property': https://github.com/lit/lit/blob/462350126d51d56f9211d50db43762d81a8b8ed1/packages/reactive-element/src/reactive-element.ts#L872
- 'no-override-get-property-descriptor': https://github.com/lit/lit/blob/462350126d51d56f9211d50db43762d81a8b8ed1/packages/reactive-element/src/reactive-element.ts#L879
- 'async-perform-update': https://github.com/lit/lit/blob/462350126d51d56f9211d50db43762d81a8b8ed1/packages/reactive-element/src/reactive-element.ts#L1354
- this one is also optional and should be documented in https://lit.dev/docs/tools/development/#development-build-runtime-warnings
- 'missing-class-metadata': https://github.com/lit/lit/blob/462350126d51d56f9211d50db43762d81a8b8ed1/packages/reactive-element/src/decorators/property.ts#L117

Either these should have the warning kind replaced with `''` in the `issueWarning()` call if the message accompanying is sufficient, or we should put these on lit.dev.

Tangentially related, I wonder if we should just have a list of all warning kinds on the site, maybe https://lit.dev/docs/tools/development/#development-build-runtime-warnings, as opposed to the /msg/{kind} urls redirecting to middle of docs.
The list of warnings can have summary descriptions. Those that warrant deep dives can then link to the relevant doc.
It's an extra point of maintenance but gives us a place to talk about the reasoning behind these warnings without finding an appropriate place in the doc to include it. It would also allow us to be more liberal about adding a warning kind instead of just `''` as we issue warning without needing to have long accompanying messages in the console.

Start with the five warning call sites linked in packages/reactive-element, then inspect the development build runtime warnings section at lit.dev/docs/tools/development/#development-build-runtime-warnings and the existing /msg/{kind} redirects. Decide whether each warning needs a redirect or an empty warning kind, and document async-perform-update as optional. Done means the listed warning kinds are handled consistently and the chosen site documentation or redirect behavior is in place.

Written by the indexing model from the issue text.