Add Markdown output support

[brandonalfred]

Is your feature request related to a problem? Please describe.
The CLI currently supports two output formats: plaintext and json. Plaintext is too minimal for reviewing flag state. JSON contains everything but is verbose. There's no middle ground that gives you a structured, readable summary at a glance.

Describe the solution you'd like
A -o markdown as a third output format. Markdown is particularly useful for LLM/AI-assisted workflows where tools consume CLI output and markdown is natively understood.

Example:

ldcli flags get --project default --flag test-feature-flag -o markdown

## test-feature-flag

Example description of what the feature flag does.

| Environment | Status | Fallthrough         | Rules |
| ----------- | ------ | ------------------- | ----- |
| beta        | ON     | Available (true)    | 0     |
| staging     | ON     | Unavailable (false) | 0     |
| production  | ON     | Unavailable (false) | 0     |

- **Kind:** boolean
- **Temporary:** yes
- **Tags:** test-tag
- **Maintainer:** John Doe

Describe alternatives you've considered
I've used -o json | jq which helps making JSON outputs pretty, but requires writing custom jq expressions.