update documentation pages

[colleenXu]

I'd like to review and update the documentation, particularly:

• https://github.com/biothings/biothings_explorer/blob/main/docs/README-writing-x-bte.md
• https://github.com/biothings/biothings_explorer/blob/main/docs/README-useful-info-for-devs.md
• Documenting x-bte extension fields #522 which is about the x-bte schema + extension docs.

There may be small edits to do in the other docs in https://github.com/biothings/biothings_explorer/tree/main/docs

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I have very rough notes saved and this may involve reviewing those notes, getting a clear list of to-dos...

[colleenXu]

Just in case, some info on BioThings parameters
biothings/mychem.info#192 (comment)

It would be nice if there was a base BioThings SmartAPI yaml that could be kept in sync with the BioThings publically-available parameters/functions @newgene