LightGBM

[docs] 📝 FAQ overhaul for linking to individual questions (#2293)

* 📝 FAQ overhaul for Issue #2268

Reformat "Contents" to use the `.. contents::` directive
Reword "Critical" into "Critical Issues"
Reformat "Critical" section to define "critical issues"
Reformat FAQ sections to follow a new format
Reformat FAQ sections so individual questions have links

All sections now follow a new format (below).
A "frequently asked question" may also include a possible
cause and a solution (if the two are not obvious from
the context):

```rst
Section Title
=============

.. contents::
  :local:
  :backlinks: none

1. Question 1
-------------

**Possible Cause**: This is likely due to...

**Solution**: Fix with...
```

* ✏️ Correcting typos and links

Add period to `2. Error messages: ....`
Fix links to FAQ in Installation-Guide.rst

* ✏️ Removing FAQ link and correcting `python-package` README

Drop general FAQ link in `Installation-Guide.rst`
Add FAQ question links to `python-package/README.rst`