start setting some max-age cache control headers on our website via htaccess [LUCENE-7986]

just by the nature of using Apache httpd our website is pretty well behaved in terms of Last-Modified & ETag headers – but diff browsers use different hueristics for how long they will cache a page before they even bother to do a validation request, that can cause many people to see "stale" pages after we do release announcements.

Example: Chrome apparently uses this hueristic – w/o any upper bound – to decide how long to keep an item in it's cache w/o revalidation...

`(date_item_was_last_fetched - last_mod_date_when_item_was_last_fetched) / 10`

...that means that if it's been 100 days since they last time we updated & published a page, when someone loads our website in chrome, their browser will cache that page for up to 10 days w/o bothering to do a cacle-validation request to see if the page has changed.

We should consider taking advantage of `mod_headers` in our htaccess file to set `Cache-Control: max-age ...` headers on various file extensions, and perhaps set lower max-ages (or must-revalidate options) on some of the pages we use specifically for annoucements & releases (ie: news, download, doc landing pages, etc...)

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Migrated from [LUCENE-7986](https://issues.apache.org/jira/browse/LUCENE-7986) by Chris M. Hostetter (@hossman)

Start by locating the website's htaccess file and reviewing how Apache mod_headers is currently configured. Identify the file extensions and announcement or release pages that need bounded caching, then confirm the intended max-age and revalidation behavior before testing the resulting response headers.

Written by the indexing model from the issue text.