Add database reference to integrations doc

[gtrkiller]

After removing the old db relation, the integration doc needs a reference to the new database relation.

[yanksyoon]

Thanks for updating the docs!

[gregory-schiano]

LGTM

[github-actions]

Test coverage for a7a8e1e

Name                Stmts   Miss Branch BrPart  Cover   Missing
---------------------------------------------------------------
src/charm.py          520     39    163     27    90%   203-206, 344-345, 557, 607, 667, 702-703, 753-759, 764, 866, 871-872, 944, 951, 1043, 1052, 1064, 1085, 1094, 1113, 1117, 1146, 1199, 1331, 1353, 1360->1362, 1401->exit, 1413, 1430-1432, 1450, 1459-1460
src/cos.py             15      0      0      0   100%
src/exceptions.py      17      1      2      1    89%   41
src/types_.py          15      0      0      0   100%
---------------------------------------------------------------
TOTAL                 567     40    165     28    90%

Static code analysis report

Run started:2023-06-29 18:13:05.609830

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 4914
  Total lines skipped (#nosec): 4
  Total potential issues skipped due to specifically being disabled (e.g., #nosec BXXX): 0

Run metrics:
  Total issues (by severity):
  	Undefined: 0
  	Low: 0
  	Medium: 0
  	High: 0
  Total issues (by confidence):
  	Undefined: 0
  	Low: 0
  	Medium: 0
  	High: 0
Files skipped (0):