[rbowen]

The Clutch status table (https://incubator.apache.org/clutch/) labels
its 18 data columns with single letters (A-R). The column name is only
available as a hover tooltip or by scrolling down to the definitions,
which makes the table hard to read.

This change:

• Replaces the single-letter header links with the full column name,
  keeping the letter in parentheses (e.g. "Start Date (A)") so the
  definitions further down the page still match.
• Applies the same labels to the repeated header row at the bottom of
  the table, so labels are visible wherever you are when scrolling.
• Adds a .clutch-table role to the table and CSS in incubator.css that
  rotates the data-column labels vertically, keeping the columns
  narrow. Podling and Sponsor stay horizontal.