document localization template interpolation, like ${0}

Nobody has claimed this yet.

please update or expand the localization docs to help those (manually) creating templates for runtime localization that show how to use interpolation and what does and doesn't work, for example a template such as `atemplate: (args)=>html'some text ${ args.named }` does not appear to work and it's unclear if the indexed approach shown below is the only option available

thanks very much.

```js
// templates has key with the template:
check_email: html`checking ${ 0 }`,
...
check_email_does_not_work: (args)=>html`checking ${ args.address }`
// unclear how to make the above templates work, except for the index approach 0, 1...
// and whether this is the only option available for logic in templates

const address = `user@host`;
msg(str`checking...${address}`, {id:'check_email'})

// like
greeting: html`Hola Mundo`,
parameterized: html`Hola ${0}`,
htmlOrderChange: html`2:${1} 1:${0}`,

```
docs at:
https://lit.dev/docs/localization/overview/

current source of information in tests:
https://github.com/search?q=repo%3Alit%2Flit%20localize%20%7B0%7D&type=code
https://github.com/lit/lit/blob/a66737fc9b861999b00ccad01edb925172b7f711/packages/localize-tools/testdata/build-runtime-xliff/goldens/tsout/zh_CN.ts#L5

Start with the localization overview at https://lit.dev/docs/localization/overview/ and compare its interpolation guidance with the referenced localize-tools test output in packages/localize-tools/testdata/build-runtime-xliff/goldens/tsout/zh_CN.ts. Document how indexed and named interpolation work, including the limitations shown by the examples, and verify that the resulting guidance answers whether other template logic is supported.

Written by the indexing model from the issue text.