Dev how-to pages (#890)

Related to #878

## Changes
How-to docs pages:

- /howto/index: home page for guides
- /howto/analytics: "How to add Google Analytics?"
- /howto/seo: "How to customize page metadata for SEO?"
- /howto/sitemap: "How to build a sitemap?"
- /howto/blog: "How to add a simple blog?"
  - blog index page
  - blog layout
  - comments -> link to the page below
- /howto/comments: "How to add user comments?"
- /howto/markdown: "How to add markdown-based content pages?"
- /howto/drd: "How to create data-rich documents with charts and tables?"

.gitignore

@@ -16,6 +16,7 @@ node_modules
 *.launch
 .settings/
 *.sublime-workspace
+.obsidian
 
 # IDE - VSCode
 .vscode/*