docs: add horizontally scale how-to

[amandahla]

Overview

This PR adds a how to about scaling Synapse up.

Rationale

Horizontal scaling is in place but we don't have a documentation about it yet.

Juju Events Changes

Module Changes

Library Changes

Checklist

• [x] The charm style guide was applied
• [x] The contributing guide was applied
• [x] The changes are compliant with ISD054 - Manging Charm Complexity
• [x] The documentation is generated using src-docs
• [x] The documentation for charmhub is updated.
• [x] The PR is tagged with appropriate label (urgent, trivial, complex)

[github-actions[bot]]

Test coverage for 4e11044e788b82f912f15384ae3141d47c9f17f8

Name                            Stmts   Miss Branch BrPart  Cover   Missing
---------------------------------------------------------------------------
src/actions/__init__.py             2      0      0      0   100%
src/actions/register_user.py       21      0      2      0   100%
src/actions/reset_instance.py      20      0      2      0   100%
src/admin_access_token.py           9      0      0      0   100%
src/backup.py                     175      5     24      2    96%   353-354, 423-424, 480->482, 483
src/backup_observer.py            134     16     14      0    89%   132-135, 140-143, 179-182, 211-214
src/charm.py                      334     17     86      9    94%   139->141, 144, 284-285, 313, 320, 401-405, 408-409, 437-439, 459, 489-492, 554-555
src/charm_state.py                133      6     34      4    94%   258, 283, 289, 295, 299-300
src/charm_types.py                 34      0     10      0   100%
src/database_client.py             57      1     12      4    93%   35, 47->exit, 69->exit, 88->98
src/database_observer.py           39      0      4      1    98%   70->72
src/exceptions.py                   3      0      0      0   100%
src/media_observer.py              41      4      2      1    88%   61-63, 82
src/mjolnir.py                     94      3     32      3    95%   82, 86->98, 108-112
src/observability.py               20      0      2      0   100%
src/pebble.py                     186     15     36     10    89%   57->62, 163->exit, 174-178, 197-198, 218-219, 272->277, 291-292, 309, 311, 313, 315, 317, 343
src/redis_observer.py              35      3      4      0    92%   62-65
src/s3_parameters.py               22      0      4      0   100%
src/saml_observer.py               31      0      6      0   100%
src/smtp_observer.py               56      4     14      2    91%   82-86, 89, 108->113
src/synapse/__init__.py             3      0      0      0   100%
src/synapse/admin.py               19      2      2      0    90%   40-41
src/synapse/api.py                175      3     24      3    97%   176, 229, 402
src/synapse/workload.py           276     26     62     12    89%   287-288, 309-310, 388->391, 394, 409->exit, 413-414, 434-435, 475-476, 512-513, 529, 576->579, 603-604, 623, 631->633, 633->635, 640-641, 656, 676-677, 694, 703-704, 723->728, 729
src/user.py                        23      0      4      0   100%
---------------------------------------------------------------------------
TOTAL                            1942    105    380     51    93%

Static code analysis report

Working... ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 100% 0:00:01
Run started:2024-07-31 08:03:12.056432

Test results:
    No issues identified.

Code scanned:
    Total lines of code: 10704
    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):