feat: Separate Short & Long Descriptions

### Is your feature request related to a problem? Please describe.

The KMD "Description" field allows the user to provide a keyboard summary of any length. While the field is useful for providing a helpful overview of the keyboard on the Keyman.com homepage, longer text is not a good fit for search results. For example, try a keyboard language search for "Amharic".

A one or two-sentence summary is more fitting for search results so that the user may view many results on a single page. More detailed description text that may include graphics, makes fewer results visible per page and likely provides too much information for the user.

### Describe the solution you'd like

The KMD "Description" field should become a "Short Description", perhaps with a string length limit. Probably, it should also be plain text. This text will be used in search results and in other contexts where a short description is suitable.

A "Long Description" field would also be added, with a subtext "For display at Keyman.com", or similar. This field will use a file browser where the user can select a Markdown (`.md`) file that contains the text (or select from the files included in the KPS). The user could, for example, select the `README.md` file for the "Long Description". The file will then be used as the content to publish at the keyboard's "homepage" at Keyman.com . The user could, of course, select another `.md` file if they wished to separate the content from the Github README.

### Describe alternatives you've considered

Continuing to enter longer overviews in the "Description" field. Search results could simply truncate after 140 chars and append an ellipsis.

### Related issues

Discussed somewhat on [#3606 ](https://github.com/keymanapp/keyboards/pull/3606)

### Keyman apps

- [ ] Keyman for Android
- [ ] Keyman for iPhone and iPad
- [ ] Keyman for Linux
- [ ] Keyman for macOS
- [ ] Keyman for Windows
- [ ] Keyman Developer
- [ ] KeymanWeb
- [ ] Other - give details at bottom of form

### Keyman version

Current

### Operating system

N/A

### Device

N/A

### Target application

N/A

### Browser

N/A

### Keyboard name

Any

### Keyboard version

Any

### Language name

N/A

### Additional context

_No response_