Update charm documentation to mention required external access

[mthaddon]

Applicable spec: N/A

Overview

Documentation change to mention required external access.

Rationale

We want to add reference documentation to each charm explaining what external access is required.

Juju Events Changes

N/A

Module Changes

N/A

Library Changes

N/A

Checklist

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

[github-actions]

Test coverage for f60c5e1

Name                 Stmts   Miss Branch BrPart  Cover   Missing
----------------------------------------------------------------
src/charm.py           201     18     41      9    89%   126-129, 157-158, 178-179, 198, 239-241, 255->266, 274-278, 352->355, 377, 390, 399->402, 477
src/file_reader.py      17      0      6      0   100%
----------------------------------------------------------------
TOTAL                  218     18     47      9    90%

Static code analysis report

Run started:2023-11-27 13:02:03.945030

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 1294
  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):