Change docs links to actual file extension (.md)
Change docs links to actual file extension (.md)

See https://gerrit-review.googlesource.com/83950 for detailed context.

Change-Id: I80bfb14b6612de133105adda707b6388d54920bf

GerritCodeReview/plugins_reviewers,davido/gerrit-reviewers-plugin,davido/gerrit-reviewers-plugin,GerritCodeReview/plugins_reviewers,GerritCodeReview/plugins_reviewers