docs: add Fikra API example to OpenAI-compatible providers

[dawn-pixellate]

Adds Fikra to the OpenAI API compatible provider list in docs/customize/model-providers/top-level/openai.mdx and adds a concrete ### Fikra API example below the generic apiBase section, matching the existing YAML/JSON Tabs format.

The example uses provider: openai with apiBase: https://fikraapi.co.ke/v1 and model deepseek-v4.1-flash.

Facts kept to Fikra's published copy, DeepSeek first:

• DeepSeek V4.1 Flash: $0.045 per 1M input / $0.09 per 1M output, 1M context
• Also served: GLM 5.3, GLM 5.3 Flash, GLM 5.2, Kimi K3, StepFun Step 5, Qwen 3.8 27B (cached input $0.10/M), Xiaomi MiMo V2.6 Pro, NVIDIA Nemotron 3.5 Super VL, plus Fikra house lanes
• Full catalog at https://fikraapi.co.ke/v1/models; keys and billing (per-token USD, $1 minimum top-up, M-Pesa and card) at https://fikraapi.co.ke

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