[boost] [website] Dates on pages are always the current date

One issue I noticed with the Boost documentation pages is that the rendered versions always have today's date on them, regardless of which Boost version the docs are for. This means that searches for Boost libraries on Google often find pages for old versions in preference to new versions; I think that changing the "Date" header to the date of the correct Boost release would make Google prefer more up-to-date docs. Is this a change that is easy to make? -- Jeremiah Willcock