NbChatComponent Overview needs more cleaner docs

### Issue type

**I'm submitting a ...** (check one with "x")

* [ x] bug report
* [ ] feature request

### Issue description

**Current behavior:**
There are no hints on NbChatComponent Overview regarding some imported files.

**Expected behavior:**
In the NbChatComponent overview, there must be cleaner docs about where the 'ChatShowcaseService' came from.

**Steps to reproduce:**

**Related code:**

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### Other information:

**npm, node, OS, Browser**
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**Angular, Nebular**
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