Create examples for right-aligned table cells

- Add numerical table cell example with right-aligned numbers
- Update table example to compare left-aligned text
- Update guidance for right-aligned table cells and link to tabular
numbers
Add left-aligned table example
Set tabular numbers by default for table cells

If the cell doesn’t contain a number, [this will fall-back to “nta”]
(https://github.com/alphagov/govuk_frontend_toolkit/blob/master/styleshe
ets/_font_stack.scss).
Remove guidance on dates
Add text alignment classes

These are generic helper classes and can be used to align data in table
cells.
Don’t default table cells to tabular numbers

This is typographically incorrect, instead use the .numerical class to
set tabular numbers

Remove text-align from numerical class
- This can be set using the helper “text-right” class
Update right-aligned table example

- Use text-right to right align text within table cells
- Use numerical for numbers which should use equal spacing to make them
easier to compare
Include both table examples in snippets page

- Add numerical table example
Scope the numerical class to table cells
Remove helper classes
Clarify usage of numerical class

The semantic difference between `th.numerical` and `td.numerical` is
significant, and gives us more control of styling without having to
give appearance classes.

Update examples to use numerical class
Remove scope="col" and add colspan="2"

Add colspan=“2”, as the heading is for both the dates and amounts table
cells underneath
Add link to pay leave for parents

Show where the example data has been taken from
Update numerical data to fit example

Show where the example data has been taken from, match data from
example on the Performance Platform.
Set numerical table cells to be right aligned
Rename numerical to numeric
Merge pull request #50 from alphagov/add-right-aligned-table-example

Create examples for right-aligned table cells