Update Template Editor Support Doc

[annezazu]

Need to update this page https://wordpress.org/support/article/template-editor/ Off the top of my head, the following needs to be thought of:

• Add info about browsing templates/site editing flows (whether this doc makes sense or a new one is needed).
• Add in mention of new theme blocks one can use.
• Add demo/video of navigating between templates.
• Explain how one can add a custom template using the template editor via post/pages but not via browsing (yet). More here: Site Editor: Add New - Custom/General template gutenberg#36860 (comment)

[annezazu]

Whew finally dove in today and did the following:

• Updated the theme block mention to link to overall blocks list as it doesn't make sense to continually update them here.
• Added in information around how there's the template editor found within the Post editor and Template Editor found within the Site Editor. I did this through creating a section titled "How to edit templates via the Site Editor" and "How to edit templates via the Post Editor".
• Added in context around where to add custom templates for posts/pages.
• Added in a demo for template editing with the site editor.
• Add demo/video of navigating between templates.

Going to ship these changes today in light of this convo in slack where current links in 5.9 RC1 are broken due to some pages not being published. Those same pages reference this one a ton so it needs to be updated at the same time.