[docs] fix missing buttons for some YSQL pages

[jaki]

Docs pages tend to have two wide buttons on the bottom of each page, referencing the previous and next pages. For instance, the Keywords page has a button to the previous Extensions page and next Reserved names page.

Things seem to get messed up when there are nested sections, namely the Statements, Data types, and Functions and operators sections. Those pages don't have any buttons on the bottom. Furthermore, the pages within each section (e.g. the CREATE TABLE page) have no previous button and only have a next button referring to the first subsection of the next section at the parent nest level (e.g. the Binary data types page).

Ideally, the buttons would refer to the immediately previous or next pages within the same nesting level. For instance, the CREATE TABLE page should have its next button refer to the CREATE TABLE AS page. Fix the buttons to be like that on at least the YSQL API docs.

[robertsami]

There are also a lot of issues like this on the architecture docs pages

a few that i've come across:
https://docs.yugabyte.com/latest/architecture/concepts/yb-master/
https://docs.yugabyte.com/latest/architecture/query-layer/overview/
https://docs.yugabyte.com/latest/architecture/core-functions/high-availability/