Code review of the presskits

When looking at the 9.6 presskit I noticed some incorrect HTML, which led me to
code review the presskits we have for related issues. The attached patch
includes the following fixes and should bring all presskits to valid XHTML:

* Multiple </a> tags closing a link

* Missing </p> tags leading to nested paragraphs on the next <p>, or one too
many </p> closing a non-existing paragraph. Also sometimes empty <p></p>
blocks.

* Incorrectly spelled tags and entities

* Missing <h1> (due to the <h1> being in the wrong place)

* Updating to use https for links to postgresql.org

* Missing alt=“” on images and unclosed <img> tags

* Missing <h2> for the Graphics section, or the <h2> inside the <a> which is
illegal in XHTML

* <li> inside a <p>, also without a leading <ul> or <ol>

It’s a rather large patch, but since it’s mainly just a lot of the same I kept
it a single commit. I can split it up into multiple patches in case that would
aid review.

cheers ./daniel