Add changelog for tracking user relevant changes

[yhaliaw]

Overview

• Add changelog under the docs directory.
• Update the PR template to include check on updating changelog.

Rationale

The users of the charm will have a place to check for updates.
Having the changelog in the docs will allow the changelog to be upload to charmhub.

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 72d7e40

Name                         Stmts   Miss Branch BrPart  Cover   Missing
------------------------------------------------------------------------
src/charm.py                   588    148    149     26    72%   237-239, 305-324, 342-344, 345->349, 375-379, 452, 459-461, 488-493, 510-516, 537, 549-555, 570-571, 584, 589, 619-620, 622->631, 626->631, 636-642, 676, 680-685, 736-741, 750->753, 776-788, 792-793, 821-848, 861-866, 885-895, 909-911, 958-959, 961-962, 964-965, 1044->1046, 1111-1112, 1150-1152, 1160-1166, 1244-1297, 1301
src/charm_state.py             452     17     96      5    95%   271-283, 508-512, 634-635, 690-691, 1126->1129, 1133-1134, 1181
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     20      0    61%   42-43, 66-69, 111-185
src/github_client.py            23      2      6      1    90%   66->exit, 71-72
src/logrotate.py                43      0      2      0   100%
src/lxd_type.py                 35      0      2      0   100%
src/runner_manager_type.py      51      0      8      0   100%
src/runner_type.py              38      0     10      0   100%
src/shared_fs.py                93     17     10      1    83%   52-53, 120-121, 146-147, 155-156, 162-163, 181, 184-185, 197-198, 241-242
src/utilities.py                32      4      6      2    79%   66-69, 111
------------------------------------------------------------------------
TOTAL                         1483    212    309     35    83%

Static code analysis report

Run started:2024-09-18 04:47:07.577175

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 4953
  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):