Update docs to remove references to k8s agent operator

[mthaddon]

Applicable spec: N/A

Overview

Trivial documentation updates, removing references to k8s agent operator.

Rationale

As above

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 1e852c7

Name                    Stmts   Miss Branch BrPart  Cover   Missing
-------------------------------------------------------------------
src/agent_observer.py      42      0      4      0   100%
src/charm.py               53      0      6      0   100%
src/charm_state.py         59      4      8      1    93%   114, 172-174
src/service.py             93      2     14      5    93%   106->118, 133, 165, 186->191, 189->186
-------------------------------------------------------------------
TOTAL                     247      6     32      6    96%

Static code analysis report

Run started:2024-02-05 11:14:42.371980

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 1056
  Total lines skipped (#nosec): 0
  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]

Thanks!