docs: add pull-request template with Motivation / Behavior / Test plan

[jacalata]

Motivation

Multiple recent reviews on this repo have surfaced the same three
questions across unrelated PRs: "why now?", "does anything change for
downstream users?", and "which claims in the body are covered by tests
vs. verified manually?" Answering them once per PR beats being asked
each time.

Behavior change

None -- adds .github/PULL_REQUEST_TEMPLATE.md. GitHub picks this up
automatically when opening a new PR against the repo.

Test plan

None -- docs-only.

🤖 Generated with Claude Code

[github-actions]

Coverage Report

File	Stmts	Miss	Cover	Missing
tableauserverclient
__init__.py	5	0	100%
config.py	15	0	100%
datetime_helpers.py	25	1	1	96%
exponential_backoff.py	20	0	100%
filesys_helpers.py	31	0	100%
namespace.py	26	3	3	88%
tableauserverclient/bin
__init__.py	2	0	100%
_version.py	358	212	212	41%
tableauserverclient/helpers
__init__.py	1	0	100%
logging.py	2	0	100%
strings.py	31	1	1	97%
tableauserverclient/models
__init__.py	46	0	100%
collection_item.py	41	7	7	83%
column_item.py	55	32	32	42%
connection_credentials.py	35	11	11	69%
connection_item.py	94	14	14	85%
custom_view_item.py	144	21	21	85%
data_acceleration_report_item.py	54	1	1	98%
data_alert_item.py	158	4	4	97%
data_freshness_policy_item.py	155	15	15	90%
database_item.py	207	36	36	83%
datasource_item.py	300	12	12	96%
dqw_item.py	104	5	5	95%
exceptions.py	4	0	100%
extensions_item.py	132	4	4	97%
extract_item.py	44	4	4	91%
favorites_item.py	69	8	8	88%
fileupload_item.py	19	0	100%
flow_item.py	149	10	10	93%
flow_run_item.py	71	0	100%
group_item.py	89	6	6	93%
groupset_item.py	49	7	7	86%
interval_item.py	182	32	32	82%
job_item.py	192	10	10	95%
linked_tasks_item.py	79	1	1	99%
location_item.py	29	2	2	93%
metric_item.py	129	13	13	90%
oidc_item.py	63	3	3	95%
pagination_item.py	34	1	1	97%
permissions_item.py	111	12	12	89%
project_item.py	207	31	31	85%
property_decorators.py	100	18	18	82%
reference_item.py	26	2	2	92%
revision_item.py	59	1	1	98%
schedule_item.py	209	6	6	97%
server_info_item.py	37	7	7	81%
site_item.py	636	13	13	98%
subscription_item.py	101	2	2	98%
table_item.py	119	18	18	85%
tableau_auth.py	61	25	25	59%
tableau_types.py	27	1	1	96%
tag_item.py	15	0	100%
target.py	6	0	100%
task_item.py	56	2	2	96%
user_item.py	310	18	18	94%
view_item.py	220	16	16	93%
virtual_connection_item.py	64	8	8	88%
webhook_item.py	69	1	1	99%
workbook_item.py	362	16	16	96%
tableauserverclient/server
__init__.py	9	0	100%
exceptions.py	4	0	100%
filter.py	29	1	1	97%
pager.py	33	1	1	97%
query.py	143	15	15	90%
request_factory.py	1335	195	195	85%
request_options.py	386	5	5	99%
server.py	188	23	23	88%
sort.py	6	0	100%
tableauserverclient/server/endpoint
__init__.py	35	0	100%
auth_endpoint.py	77	11	11	86%
custom_views_endpoint.py	152	12	12	92%
data_acceleration_report_endpoint.py	21	0	100%
data_alert_endpoint.py	94	23	23	76%
databases_endpoint.py	111	30	30	73%
datasources_endpoint.py	323	33	33	90%
default_permissions_endpoint.py	44	3	3	93%
dqw_endpoint.py	45	16	16	64%
endpoint.py	212	20	20	91%
exceptions.py	77	6	6	92%
extensions_endpoint.py	31	0	100%
favorites_endpoint.py	94	22	22	77%
fileuploads_endpoint.py	51	0	100%
flow_runs_endpoint.py	62	9	9	85%
flow_task_endpoint.py	21	2	2	90%
flows_endpoint.py	198	53	53	73%
groups_endpoint.py	126	9	9	93%
groupsets_endpoint.py	72	7	7	90%
jobs_endpoint.py	67	9	9	87%
linked_tasks_endpoint.py	37	0	100%
metadata_endpoint.py	88	14	14	84%
metrics_endpoint.py	55	6	6	89%
oidc_endpoint.py	42	1	1	98%
permissions_endpoint.py	44	3	3	93%
projects_endpoint.py	178	24	24	87%
resource_tagger.py	127	35	35	72%
schedules_endpoint.py	119	11	11	91%
server_info_endpoint.py	36	10	10	72%
sites_endpoint.py	130	27	27	79%
subscriptions_endpoint.py	56	14	14	75%
tables_endpoint.py	110	36	36	67%
tasks_endpoint.py	63	6	6	90%
users_endpoint.py	183	8	8	96%
views_endpoint.py	150	9	9	94%
virtual_connections_endpoint.py	113	10	10	91%
webhooks_endpoint.py	54	9	9	83%
workbooks_endpoint.py	338	22	22	93%
TOTAL	12007	1423	88%