Loosen Markdown syntax linters (#34)

Apply the changes made in gravitational/docs#508 and disable linting
rules for Markdown syntax that only affect formatting conventions and do
not affect user experience:

- Emphasis marker (`_this syntax_` and `*this syntax*` are both
  acceptable)
- Unordered list marker (can use asterisks and hyphens)
- Ordered list marker (can use all `1.`s or sequential numbers)

.remarkrc.mjs

@@ -34,6 +34,8 @@ const configLint = {
     "frontmatter",
     "mdx",
     "preset-lint-markdown-style-guide",
+    ["lint-emphasis-marker", false],
+    ["lint-unordered-list-marker-style", false],
     ["lint-table-pipe-alignment", false],
     ["lint-table-cell-padding", false],
     ["lint-maximum-line-length", false],
@@ -45,7 +47,7 @@ const configLint = {
     ["lint-list-item-spacing", { checkBlanks: true }],
     ["lint-no-shell-dollars", false],
     ["lint-list-item-indent", "space"],
-    ["lint-ordered-list-marker-value", "single"],
+    ["lint-ordered-list-marker-value", false],
     ["lint-maximum-heading-length", false],
     ["lint-no-shortcut-reference-link", false],
     ["lint-no-file-name-irregular-characters", false],