Update documentation for ingress library to avoid needing config for external hostname

[mthaddon]

Similarly to https://github.com/canonical/nginx-ingress-integrator-operator/pull/83 update the documentation of the ingress library to avoid the recommendation that the local charm has an external-hostname (or similar) config option that it passes to the nginx-ingress-integrator charm over the relation. Instead, default to self.app.name and let this be overridden using the service-hostname option of the nginx-ingress-integrator charm.

This means the local charm can have one less config option, but still uses the best practice of defaulting to the deployed application name.

Note that this library/relation doesn't yet implement the "ingress" relation as defined in the charm-relation-interfaces project. This update doesn't change that, but that will be addressed in future changes, at which point this library will need updating as well.

[github-actions[bot]]

Test coverage for 1bd6f6c0ddc13dfe8c1ee148c2bbc7121eb74627

Name             Stmts   Miss Branch BrPart  Cover   Missing
------------------------------------------------------------
src/charm.py       457     31    184      7    92%   415->417, 565-567, 571-576, 584-585, 593-594, 602-607, 626-628, 752->777, 1028->exit, 1086, 1129->1165, 1147-1164
src/helpers.py      10      0      2      0   100%
------------------------------------------------------------
TOTAL              467     31    186      7    92%

Static code analysis report

Run started:2023-08-08 09:32:28.216804

Test results:
    No issues identified.

Code scanned:
    Total lines of code: 3635
    Total lines skipped (#nosec): 2
    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):