Fix anchors/links in markdown [3.1.0.md]

[vksepm]

Broken link for Path Item Object

[ralfhandl]

Basically all links are now broken.

Problem is that GitHub changed the way they translate Markdown into HTML.

The OpenAPI spec uses this pattern to define explicit section anchors:

#### <aname="pathItemObject"></a>Path Item Object

This worked fine until a few months ago, since then the anchor element is no longer copied into the output HTML when it is embedded within the headline.

A systematic fix would be to change all of these header lines with embedded anchors into anchors immediately preceding the header line:

<aname="pathItemObject"></a>
#### Path Item Object

This style of anchors is still copied verbatim into the output HTML, and the links work fine.

I did this in two other GitHub repos for a bunch of Markdown files.

[handrews]

The link fixes have been consolidated for all versions in PR #3408 (and follow-up PRs as noted in its description), so I am closing all other link fix PRs in favor of that one.