Add initial documentation for reactive mode

[cbartz]

Applicable spec: ISD-116

Overview

Add a howto and some reference documentation.

Rationale

We have an initial deployment in production and need some documentation.

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).
• The changelog is updated with changes that affects the users of the charm.
• The changes do not introduce any regression in code or tests related to LXD runner mode.

[github-actions]

Test coverage for 30eb911

Name                         Stmts   Miss Branch BrPart  Cover   Missing
------------------------------------------------------------------------
src/charm.py                   630    166    140     27    70%   247-249, 315-334, 352-354, 355->359, 385-389, 463-465, 474, 481-483, 504-509, 526-532, 553, 565-571, 586-587, 606-607, 616, 621, 651-652, 654->663, 658->663, 668-674, 708, 712-717, 768-773, 782->785, 808-820, 824-825, 853-880, 893-898, 917-927, 942-944, 991-992, 994-995, 997-998, 1077->1079, 1144-1145, 1183-1185, 1193-1199, 1277-1310, 1324-1329, 1344-1387, 1395-1396, 1418
src/charm_state.py             450     17     82      3    95%   274-286, 505-509, 631-632, 687-688, 1123->1126, 1130-1131, 1178
src/errors.py                   25      0      0      0   100%
src/event_timer.py              52      6      0      0    88%   105-106, 143-144, 160-161
src/firewall.py                 51     18     10      0    67%   42-43, 66-69, 111-185
src/github_client.py            23      2      4      0    93%   71-72
src/logrotate.py                43      0      2      0   100%
src/lxd_type.py                 35      0      0      0   100%
src/runner_manager_type.py      39      0      0      0   100%
src/runner_type.py              38      0      0      0   100%
src/shared_fs.py                98     17     10      1    83%   60-61, 132-133, 162-163, 171-172, 178-179, 210, 213-214, 226-227, 270-271
src/utilities.py                32      4      6      2    79%   66-69, 111
------------------------------------------------------------------------
TOTAL                         1516    230    254     33    83%

Static code analysis report

Run started:2024-10-24 13:05:16.779961

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 5045
  Total lines skipped (#nosec): 2
  Total potential issues skipped due to specifically being disabled (e.g., #nosec BXXX): 6

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):

[erinecon]

This assumption should probably be moved to the "Requirements" section

[erinecon]

Nitpick: howto --> how-to guide

[erinecon]

For my understanding, does this how-to guide have the user deploy the webhook router?

[erinecon]

Nitpick: "to have the howto reproducible" --> "for reproducibility"