Instructions typos and small gear instructions

#266 (closed) identifies some wording and typos to correct:

• Add info link from the instructions where the tube lens is fitted to the knowledge base page on the tube lens
• imaging_optics_explanation.html last paragraph re-wording
• first page of building the high resolution microscope: 'listed our bill of materials (csv).'

Also #302 (closed)

• Add an info box to the instructions to draw attention that screws should be flush

closes: #266 (closed) closes: #302 (closed)