Update docs

[erinecon]

Overview

Small changes to the documentation that are specific to the jenkins-agent-operator charm.

Rationale

• Tutorial: Fixed outdated commands, updated headings to have consistent grammar, formatted two items as notes
• How-to guide: Fixed a fragmented sentence, capitalised Jenkins where appropriate, "arch" --> "architecture", removed the assumption about x86_64 arch and turned it into an example
• Explanation: Capitalised Jenkins where appropriate

Juju Events Changes

None

Module Changes

None

Library Changes

None

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 6060275

Name                    Stmts   Miss Branch BrPart  Cover   Missing
-------------------------------------------------------------------
src/agent_observer.py      41      0      4      0   100%
src/charm.py               62      0     10      0   100%
src/charm_state.py         58      4      8      1    92%   114, 172-174
src/service.py             98      5     14      5    91%   94-96, 108->120, 135, 167, 204->208, 206->204
-------------------------------------------------------------------
TOTAL                     259      9     36      6    95%

Static code analysis report

Run started:2024-09-06 18:55:12.837641

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 1140
  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):