docs: add changelog (ISD-2719)

[amandahla]

Overview

This PR adds a Changelog file.

Rationale

Describe and publish changes in a single file.

Juju Events Changes

Module Changes

Library Changes

Checklist

• The charm style guide was applied
• The contributing guide was applied
• The changes are compliant with ISD054 - Manging 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)

[erinecon]

Thanks for adding the changelog! One minor comment, then we should be good to go

[erinecon]

Thanks for adding the changelog!

[github-actions]

Test coverage for 8c040d2

Name                                    Stmts   Miss Branch BrPart  Cover   Missing
-----------------------------------------------------------------------------------
src/actions/__init__.py                     1      0      0      0   100%
src/actions/register_user.py               21      0      2      0   100%
src/admin_access_token.py                   9      0      0      0   100%
src/auth/__init__.py                        0      0      0      0   100%
src/auth/mas.py                            12      0      0      0   100%
src/backup.py                             175      5     20      2    96%   353-354, 423-424, 481->483, 484
src/backup_observer.py                    134     16     12      0    89%   132-135, 140-143, 179-182, 211-214
src/charm.py                              305     20     70     11    92%   140->142, 145, 258, 262-263, 269-270, 291-292, 321, 328, 408-412, 415-416, 444-446, 466, 502-503
src/charm_types.py                         30      0      0      0   100%
src/database_client.py                     57      1      8      4    92%   35, 47->exit, 69->exit, 88->98
src/database_observer.py                   49      4      2      0    92%   61-64
src/exceptions.py                           3      0      0      0   100%
src/matrix_auth_observer.py                68      8     12      3    86%   63, 66, 145, 159-163
src/media_observer.py                      45      4      2      1    89%   60-62, 81
src/mjolnir.py                            102      3     30      3    95%   85, 94->110, 115-119
src/observability.py                       14      0      0      0   100%
src/pebble.py                             210     29     46     12    84%   58->63, 177->exit, 188-192, 226-227, 247-248, 266-269, 324->329, 334-335, 347-348, 350-351, 384, 386, 388, 390, 392, 420, 474-479
src/redis_observer.py                      39      3      4      0    93%   63-66
src/s3_parameters.py                       22      0      4      0   100%
src/smtp_observer.py                       61      4     14      2    92%   82-86, 89, 108->113
src/state/__init__.py                       0      0      0      0   100%
src/state/charm_state.py                  128      9     32      7    90%   166, 170, 191, 216, 222, 228, 232-233, 316
src/state/mas.py                           72      7      4      2    88%   145, 151-152, 176-178, 193
src/state/validation.py                    36      3      2      0    92%   105-107
src/synapse/__init__.py                     4      0      0      0   100%
src/synapse/admin.py                       19      2      2      0    90%   40-41
src/synapse/api.py                        175      3     20      3    97%   176, 229, 402
src/synapse/workload.py                   138      6     24      0    94%   401-402, 412-415
src/synapse/workload_configuration.py     148     26     34     12    79%   88->exit, 92-93, 141-142, 171, 191-192, 224-225, 258, 267-268, 283, 288-289, 310-311, 330->335, 336, 354->356, 366-367, 395, 403->405, 405->407, 412-413, 433->440, 443, 463-464
src/user.py                                23      0      2      0   100%
-----------------------------------------------------------------------------------
TOTAL                                    2100    153    346     62    91%

Static code analysis report

Working... ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 100% 0:00:00
Run started:2025-01-10 12:22:09.283917

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 11328
  Total lines skipped (#nosec): 4
  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):