chore: no free form issues, 'description'-s to 'placeholders'-s (#16)

## Description

There were some problems with the roll-out of the issue templates. This PR addresses them.

## Changes

- Disable `blank-issue-enabled`.
- Move all `description` types in the issue templates to `placeholder`.

### Design Decisions

- The `description` fields take up too much space.

## Checklist

- [x] Tests pass
- [x] Docs updated

Reviewed-on: #16
Co-authored-by: M.V. Hutz <git@maximhutz.me>
Co-committed-by: M.V. Hutz <git@maximhutz.me>

.gitea/ISSUE_TEMPLATE/config.yml

@@ -1,2 +1,2 @@
 # yaml-language-server: $schema=https://www.schemastore.org/gitea-issue-config.json
-blank_issues_enabled: true
+blank_issues_enabled: false