Missing links between system catalog documentation pages

Hi hackers,

While browsing the system catalog docs earlier I noticed that a lot of
them mention other catalogs or views in the introductory paragrah
without hyperlinking them. Now, most of these are linked in the
"references" column in the table, but some, like pg_proc's mention of
pg_aggregate have no direct links at all.

The attached patch makes the first mention of another system catalog or
view (as well as pg_hba.conf in pg_hba_file_lines) a link, for easier
navigation.

- ilmari
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