Fix RSS missing content on feed readers

[boxdox]

RSS feed generated was missing content, as default zola template prefers page summary to content. Thus, in feed readers only content before <!-- more --> was being shown. Closes #255

Duplicated zola's default template, and made a few tweaks to make feed valid.

• Added rss 2.0 namespaces to allow content:encoded tags
• Author block is removed, front-matter only contains editors field, causing more conditional checks
• Description contains page's summary if available, defaulting to page title
• Content is processed for internal and relative links (replaced to 1 level only)

One post from 2022 Oct was throwing warning in w3 rss validator, so fixed it.

[boxdox]

@phil-opp looks like some external link check is failing. can you check this?