Add Discussion forms for Ideas and Show and tell

## Task

Only Q&A has a form (`.github/DISCUSSION_TEMPLATE/q-a.yml`). Default Ideas and Show and tell categories are unstructured.

## Scope

Add:
- `.github/DISCUSSION_TEMPLATE/ideas.yml` (slug must be `ideas`)
- `.github/DISCUSSION_TEMPLATE/show-and-tell.yml` (slug `show-and-tell`)

Do **not** put a category dropdown inside a category form.

## Acceptance criteria

- [ ] Filenames match GitHub category slugs
- [ ] Required body field + “searched existing threads” checkbox
- [ ] LF YAML
- [ ] No overlap with bug_report / feature_request issue templates

Start by reading .github/DISCUSSION_TEMPLATE/q-a.yml and compare its structure with GitHub's discussion form requirements. Add ideas.yml and show-and-tell.yml using the specified slugs, required body fields, and searched-threads checkboxes; verify LF YAML, no category dropdowns, and no overlap with the issue templates.

Written by the indexing model from the issue text.