Add discourse-rad-plugin to the ROCK

[amandahla]

Overview

This PR changes the Discourse ROCK by adding a new plugin discourse-rad-plugin to the list making it available to all instances of the charm.

Rationale

This is required for some discourse-based documentation to render correctly.

Juju Events Changes

n/a

Module Changes

n/a

Library Changes

n/a

Checklist

• [x ] The charm style guide was applied
• [x ] The contributing guide was applied
• [x ] The changes are compliant with ISD054 - Managing Charm Complexity
• [x ] The documentation is generated using src-docs
• [x ] The documentation for charmhub is updated.
• [x ] The PR is tagged with appropriate label (urgent, trivial, complex)

[github-actions]

Test coverage for c6ec6dc

Name              Stmts   Miss Branch BrPart  Cover   Missing
-------------------------------------------------------------
src/charm.py        314     38     86     17    86%   129, 131, 140, 142, 150-151, 291->296, 466-467, 479-481, 486-487, 498-500, 505-506, 518-520, 525-526, 538-540, 566-568, 608->exit, 615-616, 618->exit, 624, 652-658, 684->exit, 698-699, 709->exit, 723
src/database.py      30      1      8      1    95%   56
-------------------------------------------------------------
TOTAL               344     39     94     18    87%

Static code analysis report

Run started:2023-10-31 18:30:31.172847

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 1841
  Total lines skipped (#nosec): 1
  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):

[Thanhphan1147]

LGTM