Re: [whatwg/encoding] Visualization tables has lack of descriptions (Issue #292)

I actually found answers for some my questions myself during understanding the `indexes.json`, but that would be nice to have them answered on visualization pages:

"BMP coverage" pages (such as https://encoding.spec.whatwg.org/big5-bmp.html) contains a table 256x256 with following information:
- External header row (`00 01 02 ...`) is a low byte of the code point `U+__XX` in decimal form
- Internal header row contains the same but in hexadecimal
- External header column (`00 01 02 ...`) is a high byte of the code point `U+XX__` in decimal form
- Internal header column contains the same but in hexadecimal
- Each cell contains
  - Code point value at the top in form `U+xxxx`
  - Glyph at the middle or glyph for `U+FFFD` (Replacement character) if that cell does not contain any mapped code point
  - position in the index (array index in JSON array of code points) which is called [_pointer_](https://encoding.spec.whatwg.org/#index-pointer) in the specification at the bottom

The table represents `256 x 256 = 0xFFFF` characters from Basic Multilingual Plane (who would doubt).

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"Index" pages contains a tables with slightly different structure depending on the encoding with following information:
- Each cell contains
  - position in the index (array index in JSON array of code points) which is called [_pointer_](https://encoding.spec.whatwg.org/#index-pointer) in the specification
  - Glyph at the middle or glyph for `U+FFFD` (Replacement character) if that cell does not contain any mapped code point
  - Code point value at the bottom in form `U+xxxx` if cell represents a mapped value

Single-byte encodings (such as https://encoding.spec.whatwg.org/ibm866.html) contains a high half of the encoding (because they all are ASCII compatible and entries `00-7F` the same as in ASCII), so the table is always `16 x 8` and represents bytes `80-FF`:
- Header row (`00 01 02 ...`) is a low nibble of an encoded byte (`0x_X`) in hexadecimal form
- Header column (`08 09 0A ...`) is a high nibble of an encoded byte (`0xX_`) in hexadecimal form

Multi-byte encodings (such as https://encoding.spec.whatwg.org/big5.html) are more complicated. All such encodings (which are visualized) occupies 1 or 2 bytes per code point. In most cases only [ASCII code points](https://infra.spec.whatwg.org/#ascii-code-point) occupies 1 byte, so they are not included in visualization, other code points occupies two bytes:
- External header row (grey) is a low byte of the code point in the encoding (`__XX`) in hexadecimal form
- Internal header row (white) is just a row index in hexadecimal form
- External header column (grey) is a high byte of the code point in the encoding (`XX__`) in hexadecimal form
- Internal header column (white) is just a column index in hexadecimal form

Table dimensions depends on the encoding and represents constants that are used in encoding process.

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