Enhance SEO for default theme with multi-langs

[ ] Add robot.txt to config section

[x] Revise sitemap structure to including other langs. Ex:


<?xml version="1.0" encoding="UTF-8"?>
<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9" 
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.sitemaps.org/schemas/sitemap/0.9
        http://www.sitemaps.org/schemas/sitemap/0.9/sitemap.xsd" xmlns:xhtml="http://www.w3.org/1999/xhtml">
<url>
<loc>https://mixcore.org/</loc>
<lastmod>2019-12-16T16:27:32+00:00</lastmod>
<priority>1.00</priority>
<xhtml:link rel="alternate" hreflang="x-default" href="https://mixcore.org/" />
<xhtml:link rel="alternate" hreflang="en" href="https://mixcore.org/" />
<xhtml:link rel="alternate" hreflang="de" href="https://mixcore.org/de-de/" />
</url>
<url>
<loc>https://mixcore.org/top-open-source-asp-net-core-cms.html</loc>
<lastmod>2019-12-16T16:27:32+00:00</lastmod>
<priority>0.80</priority>
</url>

- [ ] Add submit sitemap auto via api if possible https://developers.google.com/webmaster-tools/search-console-api-original/v3/sitemaps/submit

- [ ] HTML tag should have lang attribute
```html
<html lang="en">

[ ] Auto gen alternate hreflang inside if other culture existing:

<link rel="alternate" hreflang="en" href="https://mixcore.org/" />
<link rel="alternate" hreflang="de-de" href="https://mixcore.org/de-de/" />
<link rel="alternate" hreflang="x-default" href="https://mixcore.org/" />

[ ] Add script config for "structured-data". Ex: https://developers.google.com/search/docs/guides/intro-structured-data
[ ] Auto add "alt" attribute to all html tag with post title in case it is blank
[ ] Add Lazyload js https://github.com/aFarkas/lazysizes/releases to site
[ ] Add rel=”next” / “prev” with a self-referencing rel=”canonical” link inside tag. So /blog/6/1 should rel=”canonical” to /blog/6/1. For example:

The code of a four-page series will look something like this:

One pagination tag on the root page, pointing to the next page in series.

<link rel="next" href="https://www.example.com/category?page=2″>
<link rel="canonical" href="https://www.example.com/category">

Two pagination tags on page 2.

<link rel="prev" href="https://www.example.com/category">
<link rel="next" href="https://www.example.com/category?page=3″>
<link rel="canonical" href="https://www.example.com/category?page=2">

Two pagination tags on page 3.

<link rel="prev" href="https://www.example.com/category?page=2″>
<link rel="next" href="https://www.example.com/category?page=4″>
<link rel="canonical" href="https://www.example.com/category?page=3">

One pagination tag on page 4, the last page in the paginated series.

<link rel="prev" href="https://www.example.com/category?page=3">
<link rel="canonical" href="https://www.example.com/category?page=4">

[ ] NO rel=”next” / “prev for 1st / last page
[ ] Auto add text to H1 like below:
[ ] Change example.com/category?page=2 over example.com/category/page-2
[x] Post detail URL format should be "/post/friendly-url-title-name-ID.html"

mixcore / mix.core

Enhance SEO for default theme with multi-langs #157