docs(migration): document individual memberships and email forwarder (#698)

* docs(migration): document individual memberships and email forwarder

Add current-state and migration write-ups for two flows being migrated
from myprofile + membership-ui into LFX Self Serve:

- Individual Memberships ($99/yr TLF Supporter): cross-app user journey,
  Stripe + Salesforce wiring, and gap analysis vs LFX Self Serve.
- Linux.com Email Forwarder ($150 lifetime alias): purchase + claim flow
  spanning myprofile, membership-ui, and the ITX mail-forwarder service,
  with UX unification as the primary migration win.

Also adjusted markdownlint to disable MD060, and add a Migration section to the
mkdocs nav.

Jira: https://jira.linuxfoundation.org/browse/LFXV2-1664

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: David Deal <ddeal@linuxfoundation.org>

* docs(migration): clarify 3-way consistency risk and migration options

Expand the 'Consistent state' row in the What needs to be built table
to name all three services that can drift (member-service, ITX,
user-service) and enumerate the three migration strategies: accept
eventual consistency, saga with compensating actions, or periodic
reconciler. Defers the design decision to the migration project.

Refs: LFXV2-1664

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>
Signed-off-by: David Deal <ddeal@linuxfoundation.org>

* fixed lint issue

Signed-off-by: David Deal <ddeal@linuxfoundation.org>

* Remove "MD060": false

The actual table issue was already fixed by Prettier. Commit b0ed816 ("fixed
lint issue") padded linux-com-email-forwarder.md table rows to consistent
widths — that's Prettier's markdown table behavior, not markdownlint.

Signed-off-by: David Deal <ddeal@linuxfoundation.org>

---------

Signed-off-by: David Deal <ddeal@linuxfoundation.org>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: dealako

.markdownlint.json

@@ -1,9 +1,15 @@
 {
   "default": true,
-  "MD003": { "style": "atx" },
-  "MD007": { "indent": 2 },
+  "MD003": {
+    "style": "atx"
+  },
+  "MD007": {
+    "indent": 2
+  },
   "MD013": false,
-  "MD024": { "allow_different_nesting": true },
+  "MD024": {
+    "allow_different_nesting": true
+  },
   "MD033": false,
   "MD041": false,
   "MD051": false