DOC Prepare for docs site

[emteknetnz]

Issue silverstripe/.github#420

This PR is preparation for adding Fluent to the "Optional Features" section on docs.silverstripe.org

This PR:

• Adds an index.md page
• Adds metadata to each page. The icons are fontawesome icons which can be seen here https://fontawesomeicons.com/
• Chanages the file names so that they display in a predictable order, using our standard naming scheme. Let me know if the order of pages appears wrong and I'll change it.
• Does some minor tidy up for standardisation with other docs e.g. uses - for bullet points rather than *

This PR will be used on the CMS 5 version of docs.silverstripe.org, and will be merged up to also be used on the CMS 6 version

[emteknetnz]

Required for composer to install correctly in CI with the new silverstripe/documentation-lint being added

[tractorcow]

Looks good to me. Nice work.

[tractorcow]

I remember the whole "numbering prefix" issue. I guess you're still using the same documentation generator? :P

[GuySartorelli]

I remember the whole "numbering prefix" issue. I guess you're still using the same documentation generator? :P

Probably 😅 It's https://github.com/silverstripe/doc.silverstripe.org/ which uses gatsby.

[GuySartorelli]

I tried to catch all the **NOTE** and **IMPORTANT** style blocks that need to be turned into callout blocks, but I may have missed some. Can you please have a quick search for any I missed?

I probably also missed some code blocks that are missing use statements

[GuySartorelli]

#981 (comment) still hasn't been responded to

[emteknetnz]

Have moved _manifest_exclude from docs/en to docs where it was originally

[GuySartorelli]

LGTM