Let user choose which version of Notify docs they want to view

When sending them a link to a particular section in the docs.

We do this as often when sending service users links to a section
in the docs, we don't know which of our clients they use, if any.

I also included "Any is fine" option to choose, for users who
aren't very technical, but would still like to see the docs.
It might be hard for these users to choose from a list of options
they are not familiar with. Hence, I provided a "comfort" option
for them. Selecting this will take them to rest API docs.

app/main/views/index.py

@@ -175,9 +175,16 @@ def guidance_api_documentation_section():
     )
 
 
-@main.route("/using-notify/api-documentation/section/choose-docs")
+@main.route("/using-notify/api-documentation/section/choose-docs", methods=["GET", "POST"])
 def guidance_api_documentation_section_choose_docs():
     form = ChooseDocsForm(section_tag=request.args.get("section_tag"))
+
+    if form.validate_on_submit():
+        redirect_url = (
+            f"https://docs.notifications.service.gov.uk/{form.docs_version.data}.html#{form.section_tag.data}"
+        )
+        return redirect(redirect_url)
+
     return render_template(
         "views/guidance/using-notify/api-documentation-section-choose-docs.html",
         navigation_links=using_notify_nav(),

tests/app/main/views/test_index.py

@@ -445,3 +445,11 @@ def test_GET_guidance_api_documentation_section_choose_docs(client_request):
         "main.guidance_api_documentation_section_choose_docs",
         section_tag="send-a-file-by-email",
     )
+
+
+def test_POST_guidance_api_documentation_section_choose_docs(client_request):
+    client_request.post(
+        "main.guidance_api_documentation_section_choose_docs",
+        _data={"docs_version": "python", "section_tag": "send-a-file-by-email"},
+        _expected_redirect="https://docs.notifications.service.gov.uk/python.html#send-a-file-by-email",
+    )